GASTEROPODA. 



97 



Shell, bent once upon itself; the two straight portions in contact. 

 Distr., 7 sp. Neocomian — chalk. Brit. France. 



Baculites, Lamarck. 

 Etym., baculus, a staff. 

 Ex., B. anceps. PI. III., fig. 13. 



Shell, straight, elongated ; aperture guarded by a dorsal process. 

 Distr., 11 sp. Neocomian — chalk. Europe, S. America (Chile). 



Baculina, D'Orb. B. Rouyana. Neoc, France. Sutures not foliated. 

 The chalk of Normandy has received the name of baculite limestone, from 

 the abundance of this fossil. 



CLASS IL GASTEROPODA. 



The gasteropods, including land-snails, sea-snails, whelks, limpets, and the 

 like, are the types of the moUusca ,• that is to say, they present all the leading 

 features of molluscous organization in the most prominent degree, and make 

 less approach to the appearance and condition of fishes than the cephalopods, 

 and less to the crustaceaus and zoophytes than the bivalves. 



Their ordinary and characteristic mode of locomotion is exemplified by the 

 common garden-snail, which creeps by the successive expansion and contraction 

 of its broad muscular foot. These mu.scular movements may be seen foUowmg 

 each other in rapid waves when a snail is climbing a pane of glass. 



The iiucleohranches are "aberrant" gasteropods, having the foot thin and 

 vertical ; they swim near the smface of the sea, in a reversed position, or 

 adhere to floating sea-weed. 



Fig. 59. A nucleobrunche.* 

 _ The gasteropods are nearly all unsymmetrical, the body being coiled up 

 spiraUy, and the respiratory organs of the left side being usuaUy atrophied. 

 In chiton and dentaliiim the branckia and reproductive organs are repeated 

 on each side. 



* Fig. 59. Carinarla cymbium, L. sp. (after Blainville), Mediterranean : p, pro 

 foscis; t, tentacles; b, branchice; s, sheD; /, foot; d, disk. ' 



