A Conchological Manual, George Brettingham Sowerby [little readers .TXT] 📗

generally the case with shells which are attached to submarine

substances, such as Spondyli, Oysters, &c.; and the degree of irregularity

will depend upon the extent of surface involved in the attachment. In the

case of fixed shells, the attached valve is usually termed the under valve,

and the other which moves freely upon the hinge, is termed the upper valve.

 

_Form and Proportions._

 

Bivalves are said to be _equivalve_ when the two valves correspond in

extent, breadth, and thickness; and of course _inequivalve_ in the contrary

case. They are _equilateral_ when a line drawn from the umbones to the

ventral margin would divide the shell into two nearly equal parts; and of

course _inequilateral_ in the opposite case, which occurs in the great

majority of instances.

 

A Bivalve is said to be _compressed_, when the distance is small from the

most prominent part of one valve to that of the other. It is _cylindrical_

when lengthened, and more or less rounded in its breadth, as in Lithodomus

(fig. 161 in the plates). It is _cordiform_ when the shape presents a

resemblance to an imaginary heart, as in Cardium cardissa (fig. 122 in the

plates), and in the Isocardia (fig. 126 in the plates). It is _linguiform_

when it resembles a tongue in shape, as in Vulsella (fig. 185 in the

plates); _rostrated_ when it protrudes at either extremity, and terminates

in a kind of point, as in Sanguinolaria Diphos (fig. 99 in the plates);

_truncated_ when it ends in a square or angle, as if cut off; an example of

which may be seen in Solen (fig. 60 in the plates).

 

Other Bivalves are distinguished as being _auriculated_, having processes

flattened and expanded on either side of the umbones, as in Pecten (cut,

fig. 82). When there is one of these on each side of the umbones, it is

_bi-auriculated_; when only on one side, it is _uni-auriculated_. When the

expansion is very broad, as in Unio alatus (fig. 142 in the plates), and in

the Hammer Oyster (cut, fig. 83), the term _alated_ is used.

 

[Illustration: Fig. 82, _auriculated_; 83, _auriculated_, _alated_.]

 

With regard to these alated species of _Uniones_, it is necessary to

observe that they are also "_adnate_," as it is termed; the two valves

being joined to each other by the dorsal edge of the expanded parts, and

united so completely in substance with each other, that they cannot be

separated without being broken. Many other terms are used to express

difference in Bivalves, but being generally applicable to Univalves and

Multivalves, as well as to them, they will be found explained at large in

the alphabetical part of the work.

 

MULTIVALVE SHELLS.

 

These are of three different kinds; first, the "_dorsal_," as they are

termed by Linnæus, because they form a ridge in the back of the animal.

They are composed of eight pieces, or separate valves, placed in a

longitudinal series, being joined to each other by inserted lamina, and

named _Articulata_ by De Blainville, on that account. The genus Chiton is

the only example of this kind of Multivalves.

 

[Illustration: Fig. 84, 85, Chiton. _a_, anterior; _p_, posterior; _d_,

dorsal ridge; _l l_, lateral areas of the valves; _c c_, central areas; _i

i_, inserted lamina; _m_, margin.]

 

The second kind, M. De Blainville terms the _lateral_ bivalves, the pieces

being placed in pairs on each side of the animal; these compose the

"Pedunculated Cirripedes."

 

[Illustration: Fig. 86, Anatina.]

 

They differ considerably in the number and arrangement of the valves; the

small ones, which are found near the peduncle in some species, are

sometimes termed accessary valves; those which form the edge through which

the bunch of Cilia protrude, are termed _ventral_, and those on the

opposite side _dorsal_. The extremity joining the peduncle is the basal, or

anterior; and the upper extremity is the apsiral, or posterior. The

peduncle is the medium of attachment to submarine substances, to which this

well known tribe of shells adhere.

 

The third kind are termed _coronular_ by De Blainville, and compose the

order Sessile Cirripedes of Lamarck; they consist of a number of valves

placed against each other side by side in a circle, supported on a plate,

or tube, or cup, and closed by an operculum composed of two or more valves.

 

The _basal support_ is sometimes thick and flat, sometimes forming an

elongated tube, and sometimes hollowed out into a cup. In other species it

is altogether wanting. The operculum always consists of more than one

piece, generally of two pairs: they are either articulated to each other by

serrated edges, and placed against each other conically, as in Balanus, or

they lie flat in two pairs against each other. Through the ventral pair the

_cirrhi_ protrude.

 

The _parietal_ valves, composing the principal part of the shell, vary in

number, form and position. The _anterior_ valves are placed on the same

side with the cirrhi; the _posterior_, those on the opposite side; and

those which remain between on each side are the lateral valves. In many

cases, particularly in Balanus, each valve is separated into the

_prominent_ and _depressed_ areas, and the inserted lamina. In some

instances, the parietal portion is formed by a single rounded piece.

 

[Illustration]

 

In the accompanying cut (87), the prominent areas are distinguished by the

letters _pr_, and the depressed areas by _r_; the posterior valves of the

operculum are marked _p. o._, and the anterior _a. o._ The basal valve

(fig. 88) belongs to a Balanus. Fig. 89 is an Acasta, the cup-shaped base

of which is represented at fig. 90.

 

In the foregoing explanations we have omitted many of those general terms

which, relating to external characters, are applicable to shells in almost

every division of the system. It may be as well, however, to enumerate a

few of them in this place, although they are explained under their

respective letters in the alphabetical part of the work.

 

When bars or ribs, or large striæ are crossed by others radiating from the

umbones, shells are said to be _cancellated_, as represented in cut, fig.

When there is a series of nodules or spines on the upper part of the

whorls, they are _coronated_, as shewn in cut, fig. 92. When a series of

projecting parts overlay each other, in the manner of tiles, as in the cut,

fig. 93, the word _imbricated_ is applied. When marked by a regular series

of ridges, radiating from the apex, they are _pectinated_; the species of

Chiton, a single valve of which is represented in cut, fig. 94, has

received the specific name of _pectinatus_, in consequence of this

character. Shells are said to be _plicated_ when characterized by angular

bendings or foldings in their surface, as shewn in cut, fig. 95. A strong

instance of this is seen in the Ostræa Crista-Galli. When the margin of any

shell has a series of minute notches, resembling the teeth of a saw, it is

said to be _serrated_; when covered with raised points or spines it is

_aculeated_; and when striated in both directions, it is _decussated_; when

covered with a number of raised rounded points, it is _granulated_; and

having a series of these points placed in a row, near or upon the edge, it

is _denticulated_, as already explained in reference to the outer lips of

Spiral Univalves. When the external surface is rendered uneven by raised

knobs, it is said to be _tuberculated_; and if rendered rough and prickly

by sharp points it is _muricated_, as in the cut, fig. 97. The term

_reticulated_ is applied to fine raised lines, crossing each other, and

resembling fine net-work.

 

[Illustration: External surface. Fig. 91, _cancellated_; 92, _coronated_;

93, _imbricated_; 94, _pectinated_; 95, _plicated_; 96, _decussated_; 97,

_muricated_; 98, _foliated_.]

 

By the foregoing general observations and explanations, it is trusted that

the reader will be prepared for the following exposition of the general

arrangement of Lamarck, and the principles upon which it is founded.

 

       *       *       *       *       *

 

GENERAL ARRANGEMENT

 

ACCORDING TO LAMARCK.

 

In Lamarck's "Histoire Naturelle des Animaux sans Vertebres," he divides

the invertebrata into classes, the 9th, 10th, and 11th of which include

animals possessed of shells properly so called. These are the ANNELIDES,

the CIRRIPEDES, the CONCHIFERA, and the MOLLUSCA.

 

The class ANNELIDES constitutes the 9th, and is divided into three orders,

namely, the "Apodes," "Antennees," and "Sedentaires"; the last of which,

_Sedentaria_, alone contains testaceous animals. This order includes

tubular shells, which, with the exception of Dentalium, are irregularly

twisted, and attached to each other, or to extraneous substances. The first

family _Dorsalia_, contains the genus Siliquaria (plates, fig. 1), known

from the Serpulæ, by the slit which passes through the whole length of the

shell on the upper surface of the tube. The second family, _Maldania_, has

the genus Dentalium (plates, fig. 2), a species of which are commonly known

by the name of "tooth shells"; these are regularly formed, curved conical

tubes, open at both extremities. The third family, _Serpulacea_, includes

the genera Serpula, Spirorbis, Galeolaria, Vermilia, Spiroglyphus, and

Magilus. The only shell that a learner would be likely to place among these

incorrectly, according to the system, is the Vermetus (plates, fig. 345),

which being regularly spiral at the apicial extremity, has been placed

among the Mollusca; to which situation the whole of the shells under

consideration have a better title than is generally supposed. It should be

mentioned that the Serpulacea are provided with opercula.

 

 

Class CIRRIPEDES.pg.1

 

Class CIRRIPEDES.

This class constitutes the tenth of invertebrated animals, and receives its

name from the jointed and ciliated branchia which protrude between the

opercular valves. They are Multivalve shells, and were all included in the

single genus Lepas in the system of Linnæus, and are commonly known by the

name "Barnacles." Lamarck has, however, divided them into two distinct

orders. First, the _Sessile_ Cirripedes or those which being composed of

several valves, joined to each other, side by side in a circle, are

attached to each other, or to submarine bodies by the basal portion of

their own substance, and form a hollow, irregular cone, with the aperture

above closed by an operculum consisting of two or more valves. Secondly,

the _Pedunculated Cirripedes_, which are composed of valves placed in pairs

against each other, so as to form a flattened disc attached by means of a

tendinous tube called a peduncle. The first of these orders includes the

genera Tubicinella, Coronula, Platylepas, Clitia, Conia, Elmineus,

Catophragmus, Octomeris, Balanus, Creusia, Nobia, Savignium, Pyrgoma, Adna,

Megatréma. The second contains the genera Pentelasmis, Scalpellum, Smilium,

Pollicipes, Bisnæus, Lithotrya, Ibla, Octolasmis, Cineras, Otion.

 

Conchological writers are not agreed as to the propriety of allowing the

above to enter into the present science.

 

Class CONCHIFERA.

 

The shell of a conchiferous animal is always bivalve, composed of two

pieces placed opposite to each other, joined at the dorsal margins by an

elastic hinge. All true bivalve shells belong to animals of this class; and

the correspondence between the shell and the animal is so true that on

examining an empty bivalve shell we can not only determine that its

inhabitant belonged to this class, but also decide on the particular order

and family in which it should be placed, without seeing the soft parts.

 

The first general division of Conchifera is that which results from

observing the muscular impressions, or marks made on the inner surface of

the valve by the insertion of the adductor muscles. All Conchifera are

divided into two orders, as follows:

 

First Order, _Conchifera Dimyaria_.

 

Having two adductor muscles, and consequently two impressions in each

valve. They are separated into the following families:

 

_Tubicolæ_ (plates, fig. 44 to 54), having shelly tubes besides the

   valves. This family contains the genera Aspergillum, Clavagella,

   Teredina, Teredo, Xylophaga, Fistulana, and Gastrochæna.

 

_Pholadaria_ (plates, fig. 55 to 59), cylindrical, living in holes

   in rocks pierced by the animals. Lamarck places in this family the

   genera Pholas and Gastrochæna, the last of which belongs more properly

   to the family Tubicolæ, as placed above.

 

_Solenacea_ (plates, fig. 60 to 68), longitudinally (transversely,

   Lam.) elongated, open at the anterior and posterior extremities. This

   family contains the genera Solen, Pholadomya, Panopæa, Glycimeris

   (Solecurtus) and Solenimya.

 

_Myaria_ (plates, fig. 69 to 76), ligament internal. A spoon-shaped

   ligamentary pit in one or