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

regular

spire only taking place after the few first volutions.

 

SPIRAL NON-SYMMETRICAL UNIVALVES.

As these constitute the largest class, it will be necessary to dwell upon

them in detail. First as to _measurement_.

 

[Illustration]

 

The length is measured from the apex, to that part of the aperture _a_

(fig. 24), at the greatest distance from it. The _breadth_ is in the

opposite direction. The _anterior_, or front part of the aperture, is

marked _a_, where the head of the animal protrudes.

 

_Spire of non-symmetrical Univalves._

 

[Illustration: Fig. 25, _obtuse_; 26, _acute_; 27, 28, _decollated_; 29,

_concave_; 30, _papillary_; 31, _mammellated_; 32, _discoidal_.]

 

In counting the whorls of which the spire consists, we commence at the

apex, and reckon downwards to the last, or body whorl. The spire is

described as being long or short in relation to the aperture: in which

case, all that is above the aperture is measured with the spire. Its apex

requires particular notice, as the character of the whole shell frequently

depends upon the particulars observable in this part. It is sometimes

_obtuse_, or blunt; sometimes _acute_, or sharp. In the Cones it is

frequently flat, and in Planorbis it is concave. It is sometimes of a

different structure from the rest of the shell, retaining the horny and

transparent appearance which characterized it when the animal was first

hatched. The Tritons present an instance of this, although it is not always

observable, owing to the tenderness of the substances which causes it to

break or fall away in many specimens. A very remarkable instance also

occurs in Bulinus decollatus (cut, fig. 27, 28), so named, because the

apex, to the depth of several whorls, falls off, and the shell is

_decollated_. In this, and many more instances, among Pupæform land shells,

the occurrence of this circumstance seems to be by no means rare or

accidental, a provision having been made for filling up the opening by a

septum. A _papillary apex_ is one which is swelled at the extremity into a

little rounded nob, or nipple; and a _mammellated_ apex is one which is

rounded out more fully into the shape of a teat.

 

_Whorls._

 

The spire is described as consisting of _numerous_ or _few_ whorls, and

sometimes the number of them is particularly stated. A whorl consists of

one turn of the spiral cone. The whorls are described as _flattened_, when

the sides are not bulged out so as to cause the outline of the spire to

deviate considerably from straightness: when the contrary is the case, the

whorls are said to be _ventricose_, and either _rounded_ or _angulated_.

The degree of rapidity with which the whorls become enlarged presents an

important source of distinction. The _suture_, or seam, which separates one

whorl from another is also noticed as being _distinct_ or otherwise;

_canaliculated_, or grooved; or covered by an enamel, which in some

instances is swelled into a ridge or _tumid_.

 

[Illustration: Fig. 33, _few_; 34, _numerous_; 35, _rounded_, _ventricose_;

36, _angular_, _ventricose_; 37, _flattened_.]

 

_Suture._

 

[Illustration: Fig. 38, _canaliculated_; 39, _enamelled_.]

 

_Varices._

 

Varices are caused by periodical rests or stoppages in the growth of the

shell, when the edge of the aperture thickens, and renders the shell as

complete as when full grown. Again, after an interval, another check takes

place, and another thickened edge is formed, and so on in succession, until

the animal arrives at maturity, and the shell is full-grown. The thickened

edges successively forming the aperture, remain visible on the outside,

through all the subsequent stages. When these rests take place at frequent

periods, the varices will of course be numerous as in Harpa and Scalaria.

They occur at regular or irregular distances, varying in shape and other

characters. When the varices occur at regular intervals, and form a

connected ridge from whorl to whorl up the spire, they are said to be

_continuous_, as in Ranella; when on the contrary, the varix on one whorl

does not come in contact with that on the other, they are described as

_discontinuous_. In order to distinguish a regular varix from a mere

external ridge, it will be sufficient to notice whether its edge overlaps

the external surface, and whether it resembles the open edge of the

aperture, which true varices do.

 

[Illustration: Fig. 40, _numerous_; 41, _few, continuous_; 42, _few,

discontinuous_.]

 

_Aperture._

 

The aperture or opening of the spiral tube, was formerly described as the

mouth; a term calculated to convey an erroneous impression, when applied to

a part of the shell which has no correspondence with the mouth of the

animal. The word _aperture_ is used by modern writers in a general sense,

including the cavity, its edge, and the canals. The cavity itself is

distinguished in various shells as to its shape, which depends much upon

the degree of modification produced by the last whorl. In some cases, as in

Cyclostoma, where the aperture stands apart from the last whorl, the shape

is round, or nearly so. The Scalaria presents a good example of this. In

others, where the inner edge or lip, wrapping over the body whorl is nearly

straight, the aperture is _semi-lunar_, or half-moon shaped: this is

remarkable in the "_Neritacea_" of Lamarck, named, on that account,

"_hemi-cyclostomata_" by De Blainville. In a great number of instances, the

lower part of the body whorl enters obliquely into the upper part of the

aperture, the result being a _pyriform_, or pear-shaped opening. The

aperture is described as _long_ when it is largest in the direction of the

axis, and _wide_, in the contrary case. The _anterior_ is the part at the

greatest distance from the apex, and the body whorl; the _posterior_, the

part nearest to the apex. Thus some apertures are described as _posteriorly

contracted_ and _anteriorly widened_, or the reverse. A _linear_ aperture

is one contracted in its whole length, as in Cypræa. When the whorls are

angulated, a _trigonal_ aperture is the result, as in many species of

Trochus. Some are _transversely oval_, that is in an opposite direction to

the axis, and others _longitudinally oval_. When the whorls are formed with

two outer angles, a somewhat quadrated aperture is formed. There are other

variations too numerous to mention.

 

_Apertures._

 

[Illustration: Fig. 43, Helicina, _semilunar_; 44, Pirena, _pyriform_; 45,

Cypræa, _linear_; 46, Trochus, _trigonal_; 47, Cyclostoma, _rounded_; 48,

Chilina, _posteriorly contracted_; 49, Stomatia, _transversely oval_; 50,

Murex, _longitudinally oval_.]

 

The entire edge of the aperture described generally, is named the

Peritrême, but this term can only be conveniently applied in cases where,

in some at least of its characters, it is the same all round, so that one

descriptive term is applicable to the whole. As, however, this is of rare

occurrence, it is found convenient in descriptions to separate the rim from

the outer lip. In a great number of instances, this is done naturally, by a

canal, or notch at the anterior or lower extremity, and by the posterior

union of that part which overlays the body whorl with the other portion. At

these two points the outer and inner lips separate from each other: we

therefore describe the

 

_Canals of the Aperture._

 

When there is neither notch nor canal, anteriorly or posteriorly,

interrupting the edge of the aperture, it is described as entire. When

there is a notch or sinus at the anterior extremity, it is said to be

_emarginated_. When the edge of this notch is expanded, and drawn out in

the form of a beak, it is said to be _canaliferous_, or to have a _canal_.

When, in addition to this, the lips are thickened and contracted

posteriorly near their junction, and drawn out so as to form a groove, it

is said to be _bi-canaliculated_, or to have two canals. The _anterior

canal_ is said to be long or short, according to the proportion which it

may bear to the rest of the shell. Thus the canal of Ranella ranina (fig.

393 in the plates), may be described as _short_; while that of Murex

haustellum, (fig. 396, pl.) is _long_. When it is wide near the aperture,

and becomes gradually contracted towards its termination, it is said to be

_tapering_, as in Pyrula (fig. 388, pl.); when the termination is sudden,

it is described as _truncated_. If, on placing the shell upon a plane, with

the aperture downwards, the canal is seen to rise upwards, it is

_recurved_. In Buccinum and Nassa it is turned suddenly over the back, and

forms a short, curved elevation; it is then described as _recurved_ and

_varicose_. If the edges meet, so as to form a tube, it is said to be

closed, as in some species of Murex and Typhis. The posterior canal is, in

some cases, _free_, or standing out from the spire, as in some species of

Ranellæ; while in others it is _decumbent_, running up the sides of the

spire, as in Rostellaria (fig. 402, pl.).

 

_Canals._

 

[Illustration: Fig. 51, Fasciolaria, _truncated_; 52, Nassa, _recurved_,

_varicose_; 53, Cerithium, _recurved_; 54, Typhis, _closed_.]

 

_Lips, or edges of the Aperture._

 

The part of the edge of the aperture next to the body whorl is named the

_inner_, or _columellar_ lip. Posteriorly it commences at the point of

union with the outer lip, where that touches the body whorl, the junction

being generally marked by an angle, and sometimes by a canal. Anteriorly it

terminates where there is generally seen a notch or canal, or sudden angle,

from which the outer lip proceeds. The part which setting out from the body

whorl, and proceeds outwards at a distance from the axis, till it reaches

the anterior canal or notch (or its place in case of absence) is named the

_outer lip_. In many cases the edges are united in such a manner, that it

is difficult to distinguish where the inner lip terminates, and the outer

lip commences: when this is the case, it is usual to describe the margin or

peritrême, as a whole, without distinguishing the parts. The _outer_ lip,

sometimes called the right lip, or _labrum_ of continental writers, is

sometimes acute, not being of thicker substance than the remainder of the

shell. In other cases it is _obtuse_, or thickened and rounded at the edge.

When thickened and turned backwards it is described as _reflected_; when,

on the other hand, it is turned inwards towards the axis, as in the

Cyprædæ, it is _inflected_, or involute. When it is _toothed_, a

distinction must be observed as to whether the dentations are external or

internal. If the teeth are small and numerous, it is _denticulated_; if

larger, it is _dentated_; when expanded into a kind of wing, as in some

species of Strombus and Rostellaria, it is described as _alated_; and a

family in Lamarck's system is named "Alatæ," from this very circumstance.

In some of those which are expanded, the expansion is divided into

separate, attenuated portions, they are then said to be digitated.

 

_Outer Lips._

 

[Illustration: Fig. 55, Helix, _reflected_; 56, Cypræa, _involute_,

_denticulated_; 57, Sera, _alated_; 58, Murex, _digitated_; 59,

Rostellaria, _dentated_.]

 

The _inner_ lip, sometimes named the _columellar_ lip, or "_labium_," is

subject to similar variations as to thickness, dentition, &c. That portion

of it which lies upon the body-whorl is frequently distinguished from that

which intervenes between it and the notch or canal. De Blainville,

restricting the term _lip_ "bord gauche" to the former portion, applies the

term "columella" to the latter; and in some instances this may be the more

convenient method of describing the part in question. The columellar lip is

sometimes _detached_ entirely from the body of the shell, as in Murex

haustellum; in others it is _decumbent_, or lying over the last whorl,

although quite distinct, and in some cases, _thickened_, _callous_, or

_tumid_.

 

At the lower or anterior part, sometimes called the _columella_, there are

in many instances flattened, laminated folds; these are particularly

conspicuous in the genera Cymba and Melo, where, being obliquely spiral and

laminar, they are extremely elegant, presenting to the eye graduated

repetitions of the line of beauty. In other cases, as in the Turbinellæ,

they are more horizontal and thickened.

 

In some cases the columella is swelled into a varicose mass; as in Oliva,

Ancillaria, &c.; it is then described as _tumid_ or varicose. It is

sometimes _tortuous_, and sometimes straight, and is