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CLASS MOLLUSCA. 



SUB-CLASS G ASTEROPODA. 



ORDER PECTINIBRANCHIATA. COMB-GILLS. 

 THIS Order is the most extensive of the several Orders into which the 

 Class Gasteropoda is divided ; it embraces many of those animals whose 

 shells are conical, and all which have spiral univalve shells. It constitutes 

 the Paracephalophora Dioica of Blainville. 



The term applied to this Order is incorrectly written on our Plate : in- 

 stead of Pfenobranchiata, it should read Pecrintbranchiata. 



THE TRICHOID TRIBE. 

 ILLUSTRATIVE EXAMPLES. 



PLATE 6. 



MONODONTA. Shell oval or conoid ; aperture entire, rounded, with the 

 border disunited above; columella arched, and truncated at the base; 

 aperture with an operculnm. 



Type of the genus, M. pagodus, Lamarck. There are several species, 

 all marine shells, inhabiting the tropical seas. 

 Illustration : Animal of Monodonta. 



TROCHUS. Shell conical, with an elevated spire, sometimes shortened ; 

 its periphery more or less angular, often delicate and sharp ; aperture 

 transversely depressed; its edges disconnected above; columella arched, 

 more or less projecting at the base ; an opercule. Animal, gasteropodons ; 

 spiral ; head furnished with two tentacules, having subpeduncular eyes at 

 their base ; tongue armed with booklets ; respiratory cavity not furnished 

 with a syphon. 



The animal of this genus is very similar to the Murices, but distinguished 

 by the absence of a syphon at the front of its respiratory cavity for the 

 introduction of water, and by the edges of its cloak being furnished with 

 lobes. Although the aperture is more or less square, the opercule is 

 always completely circular, slightly convex where attached, and concave in 

 the opposite direction. They are marine animals, living at a short distance 

 from the coast, in clefts of rocks, or wherever marine plants, corallines, 

 &c., are found. They are distributed throughout all seas. Gmelin enume- 

 rated eighty species, and though abstracting some for the formation of his 

 genera Solarium Rotdla, Lamarck still describes seventy-eight. Trochus 

 Imperialis, Gmel., may be taken as the type. 

 Illustrations : Trochus Henslawii, T. Emma. 



This genus has been composed of some of the shells of the 

 The whorls are angulated and stellated. 

 Illustration : Imperata Gibberosus. 



PALUDINA. Shell conical, with the whorls convex ; aperture rounded, 

 ovate, longer than broad, and angular at the apex ; pillar-lip simple ; 

 margin not reflected ; operculum horny, orbicular. 



Type of the genus, Helix vivipara, Linn. The species of 

 this genus generally inhabit the fresh waters of Europe, and 

 bring forth their young alive ; the animal of the type has two 

 linear subulate tentacula, with eyes at the base ; the mouth 

 is triangular ; the foot nearly triangular ; and the branchia 

 are composed of tufted filaments. 



Illustrations : Paludinafasciata, P. Costata. 

 Shell conical, of a somewhat turricular shape ; its periphery 

 never compressed; aperture round, perfect, its edges distinct above; 

 columella arching, flattened, its base not truncated ; furnished with an 

 opercule. 



The shell of this genus is solid, often very thick and agreeably diversified 

 with brilliant nacrous colours; its twists are always round, never com- 

 pressed or sharp. The foot or ventral disc of the animal is shorter than 

 its shell, and is obtuse at both ends : the head is furnished with a pair of 

 pointed tentacules which support the eyes at the outside of their base. 

 Lamarck enumerates thirty-four species, of which seven are European. 

 T. Littoreus, Lin., may be taken as the type. 

 Illustration : Operculum of Turbo. 



LITTORIXA, a genus of spiral, univalve, marine, or littoreal shells, allied 

 to Nerita, indicated but not characterised by Ferussac. Shell univalvu, 

 solid; spire short, conic, or retnse; mouth roundish, entire; inm-r lip 



IMPERATA. 

 genus Trochus. 



Paludina. 



TURBO. 



flattened, outer lip thin ; operculum horny, free, spiral ; spire of four 

 whorls; the animal pectenibranchous ; tentacula two, compressed, re- 

 tractile ; eyes sessile at the outer base of the tentacula. 



The type of the genus is L. littoreus, the Turbo littoreus of Linnseus, the 

 common Periwinkle. 



The species of the genus are numerous, and found on the shores of most 

 parts of the world. The animals have the faculty of living a considerable 

 time out of the water. 



Illustration : Littorina littoralis. 



PHASIANELLA. Shell conical, solid ; aperture entire, lengthened, con- 

 tracted by the projection of the body whorl ; columella smooth, attenuated 

 at the base; peristome incomplete; operculum calcareous ; animal with two 

 long conical tentacula, the eyes supported on pedicles at their base. 



Type of the genus, Helix polita, Montagu. Of this beautiful genus, four 

 recent and a few fossil species have been discovered in Britain ; others 

 inhabit the Indian and Southern Oceans. 



Illustration : Phasianella bidimoides. 



AMPULLARIA. Shell roundish, ventricose ; spire short ; aperture high ; 

 operculum calcareous; columella umbilicated. Animal not unlike the 

 common pond snail (Paludina) ; tentacula long and slender ; eyes pednn- 

 culated ; air or water pouch (which is the chief peculiarity of this genus) 

 at the bottom of the respiratory sac, and at the side of the long branchial 

 comb. 



This genus forms a division of Blainville's Ellipsostomata, which is 

 defined as a group of spiral univalve shells, containing the genera Melania, 

 Risson, Phasianella, Ampudaria, Helicina, and Pleurocerus. The genus is, 

 however, quite artificial, and contains land, fresh-water, mid marine Mbllusca, 

 breathing by two kinds of respiratory organs. 



Illustration : Ampullaria solida. 



MELANIA. Shell fluviatile, operculate, turreted; aperture entire, oval 

 or oblong, widened at the base, pillar smooth, arched within ; operculum 

 horny; animal respiring only water by protruding branchiae ; phytiphagous, 

 with two tentacula; its hinder extremity spirally convolute, enveloped in 

 the shell. All the species are exotic. 



Illustration : Melania Bironensis. 



NATICA. Shell operculated, subglobose, umbilicated, aperture entire, 

 semiorbiculate ; pillar lip entire, not entering the aperture, with a cavity or 

 callus behind, the latter varying the form of the umbilicus, and occasionally 

 closing it. 



Type of the genus, Nerita glaucina, Linmeus. Rather a numerous 

 genus, consisting entirely of marine shells, adorned with beautiful colours, 

 and smooth ; eight recent, and several fossil species are found on the coasts 

 and crags of Britain. 



Illustration : Natica Millepunctata. 



NERITA. Shell solid, semiglobular, flattened below, not umbilicated, 

 furnished with an operculum ; aperture entire, semicircular ; inner lip 

 flattened, margined, often dentated ; the teeth or crenulations on the inner 

 face of the outer lip. 



Type of the genus, N. littoralis, Linnsus. 



The Neritce are all marine shells, and some of them are very prettily 

 coloured ; there are many species, only three of which inhabit the shores of 

 Britain. The type is the common Periwinkle. 



Illustrations : Nerita undulosa, N. cariosa. 



NERITINA. Shell thin, semiglobular or oval, flattened below, not umbi- 

 licated, furnished with an operculum ; aperture semicircular, the left margin 

 flattened; no teeth or crenulations in the internal face of the outer lip; 

 operculum with a lateral point. 



Type of the genus, N. Jtuviatilis, Linnteus. 



The Neritinae are river shells; the type is uluiiidant in the rivers of 

 England, especially in the Thames. 



Illustration : Neritina Ovenii. 



