GASTEROPODA. L'57 



No. (305. G-yroceras Eifelense, D Archiac. 



From tlie Devonian of the Eifel, Rhine Valley. The origi- 

 nal is in the Ward Museum of the University of Rochester. 



Price. $0.40. 



No. 606. G-yroceras nereum, Hall. 



FTom the Corniferous Limestone (Devonian), Auburn, N. T. Price, $0.7o. 



No. 607. G-yroceras undulatum, Vanuxem. 



This shell is nautiloid ; whorls separate ; and the siphuncle, eccentric and 

 radiated. This perfect specimen, with, free coils, beautifully exemplifies the 

 genus. From the Corniferous limestone (Devonian), Schoharie Co, New York. 



Price, $0.73. 



CLASS II.— GASTEROPODA. 



These " belly-footed" Molluscs are the types of the Sub-kingdom. 

 They make less approach to the Fishes than the Cephalopods, and less 

 to the Crustaceans and Zoophytes than Bivalves. The characteristic 

 mode of locomotion is exemplified by the common Snail, which crawls 

 by the alternate expansion and contraction of its foot. Gastropods are 

 terrestrial or aquatic ; bisexual or hermaphrodite ; oviparous or ovo-vivi- 

 parous. The body is generally nnsymmetrical • and. with few exceptions, 

 the visceral part is coiled up in a shell usually spiral and univalve. The 

 apex of the cone (save in the Xucleobranchs,) is always oblique and ec- 

 .eentric : this departure from a perfect spiral is owing to the unequal 

 development of the two sides of the animal. The spirals are right- 

 handed; rarely otherwise. Xo shell is multilocular like the Ammonites. 

 The muscular impression is crescent-shaped. The operculum is equiva- 

 lent to the dextral valve of Lamellibranchs. 



The Class is divided into four Orders, distinguished chiefly by the 

 form and position of the lungs. The living species have a world-wide 

 range, inhabiting the bottom and surface of the sea, sea shore, fresh 

 water and dry land ; 8,000 are vegetable-feeders ; 5.000. animal-feeders. 

 The fossil species number over 9,000. They are found in every fossilifer- 

 ous rock, except the ' : Lingula flags." Those which first appear in the 

 Paheoz«ic strata have entire mouths ; the siphonated species are not found 

 lower than the Lias, and they go on, increasing in number in and from 

 the Tertiary series to actual sea shores. The remains of Gastropods are 

 highly important to the Geologist, as they afford him unequivocal evi- 

 dence of the terrestrial, fluviatile, lacustrine or marine condition under 

 which strata were formed. 



