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. 

 Dlstr., 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 

 lie abundance of this fossil. 



CLASS II. GASTEROPODA. 



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

 ke, are the types of the mollusca ; that is to say, they present all the leading 

 ;atures of molluscous organization in the most prominent degree, and make 

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

 id less to the crustaceans and zoophytes than the bivalves. 



Their ordinary and characteristic mode of locomotion is exemplified by the 

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

 its broad muscular foot. These muscular movements may be seen following 

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



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

 irtical ; they swim near the surface of the sea, in a reversed position, or 

 ihere to floating sea-weed. 



Fig. 59. A nucleobranche.* 



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

 irally, and the respiratory organs of the left side being usually atrophied, 

 i chiton and dentalium the branchite and reproductive organs are repeated 

 i each side. 



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

 scis; t, tentacles; b, branchiae; s, shell; /, foot; d, disk. 



