130 



CLASS-MOLLUSC A. 



SUB-CLASS GASTEROPODA. 



ILLUSTRATIVK EXAMPLES. 

 PLATE 2. 



NODOSARIA. Shell elongated, straight, or a little bent, subconical, noJose, 

 with bulgings at the place of the cells ; transverse septa perforated. 



Type of the genus, N. radiada. Inhabits the Adriatic Sea. 



Illustrations : Nodosaria kevigata, Margintdiiia raphanus, Pawnia flabel- 

 liformis. 



TEXTULARIA. The genus Textularia consists of Microscopic Forami- 

 nifera, having numerous alternate chambers with marginal aperture. The 

 wall of each turn is complicated, extended, and united on each side to the 

 other discs. 



Illustrations : Pdymorphina commutes, Textularia aciculata. 



ROT ALIA. Shell orbicular, spiral, convex, or conical above ; flat, radi- 

 ated, and tubercular beneath, many-celled ; aperture marginal, triangular, 

 resupinate. 



Type of the genns, Nautilus Beccaria. Several species (minute) are 

 found on the coasts of Britain ; others of a larger size occur in a fossil state. 



Illustrations : Uvigerria pygmea, Bulimina marginata, Rosalina globularis, 

 Truncatulina refulgens. 



POLYSTOMELLA. Shell disciform, multilocular, with contiguous turns 

 not visible externally, and marked with furrows traversing the whorls ; 

 aperture composed of several holes variously disposed. 



Found only in a fossil state. 



Illustrations: Plamulina araminensis, Dendritina arbuscula, Nonionina 

 vmbilicata. 



MIUOLA. Shell ovate-globose or elongated, transverse, multilocular, 

 with transverse cells surrounding the pillar and alternately covering each 

 other ; aperture very small, situated at the base of the last whorl ; orbicular, 

 or oblong. 



Chiefly fossil; minute, occurring abundantly in the works of certain 

 quarries near Paris, and in a recent state on fuci, near Corsica. 



Illustrations: BUoculina buttioides, Triloculince trigonula, Qumquekculina 

 saxonun. 



ORBICULINA. Shell subdiscoid, multilocular, with compound, contiguous 

 turns ; spire eccentric ; cells short, very numerous ; septa imperforate. 



A fossil genus, containing several species, none of which are British. 



Illustrations : Amphegistina Qyoii, Heterogystina depressa, Tabularia dis- 

 aoHtte. 



SUB-CLASS GASTEROPODA. BELLY-WALKERS. 



THE whole of Cuvier's Cyclobranchiato, Scutibranchiate, Tubulibranchiate, 

 and Pectinibranchiate Gasteropods, and also both the Aquatic and Terrestrial 

 Pulmonibranchiate Gasteropods, excepting the single genus Vaginulus, are 

 furnished with a single or univalve shell, placed upon the dorsal surface of 

 the body, and varying remarkably in its position and size : thus in Parmacella 

 it is shallow and small, and on the middle of the back ; in Testcuxlla 

 (Plate 3) also small, and on the hind part of the back ; and in Limax 

 (Plate 3) on the fore part of the back, but remarkable in this genus as 

 being concealed by the skin : in all three genera it forms a vault over the 

 pulmonary cavity. Other and larger shells, as the Sea Ear, Haliotis, cover 

 almost entirely the whole dorsal surface of the animal. But in a very large 

 proportion of the Gasteropods, the organs of nutrition and reproduction are 

 always contained within a capacious conical or tubular shell, and even the 

 head and entire foot can often be retracted within it, as in the Snail. 



The soft exterior covering of Gasteropods is divided into two distinct 

 parts, the foot and the visceral bag. 



The Foot consists of a soft expanded tegument, containing within it a 

 large mass of longitudinal muscles passing from one end to the other, and 

 occupying its ventral surface. Its upper surface, when expanded in crawling, 

 forms a longitudinal hollow, which is perfected into a tube by a skinny 

 arch also, and which sometimes overlaps the foot like a fringe. This is 

 generally but too loosely called the mantle, from its supposed correspondence 

 to the mantle lining Bivalve Shells ; and sometimes designates all the upper 



or dorsal surface of the animal which can be protruded from the mouth of 

 the shell, but at other times is restricted to the shield-like piece which 

 covers the lung of the Pulmoniferous Gasteropods, and either, as in the 

 Slug, includes the shell, or, as in Parmacella and Testacetta, has the little 

 shell resting upon it. From its fore part the head protrudes, and seems, as 

 in the Snail, to consist merely of processes of this dorsal tegument. In 

 Vaginulus, Testacetta, Parmacella, and Limax, the dorsal tegument is j>orfect ; 

 but in most other Gasteropods it is deficient either far forwards, as in the 

 Trachelipods, or farther back, as in the Limpets. Around this aperture is 

 attached the Visceral bag, membranous and varying in form, conical or con- 

 voluted, as in the Limpet and Snail, but distinctly corresponding with the 

 muscular visceral bag of the Cephalopods. Around the junction of the 

 visceral bag with the dorsal surface of the foot, a loose sort of membranous 

 girdle, enclosing the apparatus for secreting the shell, is placed ; this is the 

 collar, and analogous to the mantle of Bivalve Shells. 



The primary form of all Univalve Shells is resolvable into that of a 

 simple hollow cone, of which the top is the tip or first-formed part of tin- 

 shell, and the base the last formed, which continually grows and forms the 

 open area of the cavity of the shell, the walls of which, included between 

 the base and the tip, are called the body of the shell ; these circumstances 

 are well exemplified in our Common Limpet, Pati-H<i \'n.l<iata. But from 

 this simple condition the cone gradually varies in different kinds of shells, 

 both in the comparative dimensions of its parts and in the direction which 

 its base, or recently-formed part, takes, whence arise the diversified forms 

 of shells. The technical name of the convolutions of shells is wliorls, 

 anfractus. In most instances shells, instead of revolving in the same plane, 

 and acquiring a disc-like form (hence called Discoid), grow obliquely 

 forwards, from right to left, so that the tip of the shell, whence the growth 

 had commenced, is generally to the right and above, and the aperture to 

 the left and below ; sometimes, however, the growth is from left to right, 

 and hence the terms right and left, or dextral and sinistral Shells. All 

 such Shells are said to form spires, which consist of all the whorls between 

 the tip and the aperture of the shell ; and in proportion as the whorl 

 flatter, wider, and shorter, as in the Helix Algira (Plate 3), or rounder, 

 narrow, and longer, as in the Scalaria, the shell is said to be turbinated or 

 spiral. Sometimes, as in the semirevolute Shells, the whorls are perfectly 

 distinct from each other, of which the False Wentletrap, Scalaria communis, 

 furnishes an example ; but more commonly they rest against each other, as 

 in the Snails. 



The transition, in all its varieties, of different Shells from one form to 

 another, the difference of shape in the Shells themselves, and of their 

 different parts, and the peculiar forms assumed by the margin of the aper- 

 tures of Shells, and the direction which they take, are too numerous to be 

 considered here, although they form very important characters of the several 

 kinds. But it must be observed, that the interior of Univalve Shells 

 generally consists of a single cavity, and such are called Unilocidar or 

 Monothalamous. In some, comparatively few, however, the cavity is divided 

 by less or more perfect partitions ; such are called Chambered Shells ; and 

 if the number of the chambers be many, they are named Multilocular > >r 

 Polythalamous, of which the shell of the Pearly Nautilus, Nautilus Pomjiilitts, 

 a Cephalopod, affords a good example. The connection of an Univalve 

 Shell to the animal which it partially or wholly contains, is by its collar 

 just within the aperture; but in addition to this, muscular fibres also pass 

 from it to the foot, and, as in the Snail, from the lowest or basal extremity 

 of the columella. 



ORDER PULMONIFERA. AII:-HI:KATHERS. 



THE arrangement of the Gasteropods rests almost entirely upon the con- 

 ditions of their respiratory organ : the greater number, being aquatic, are 

 furnished with gills, branchiae, whilst the remainder have lungs, pulmones, 

 and either live entirely on the earth's surface, or, living in water, are com- 

 pelled to rise to its sin-face, in order to inspire fresh air. Hence the 

 Gasteropods are divided into Branchiferous and Pulmoniferous ; the Com- 



