PALUDINA. LITTORINA. MONODON. 



49 



Fig. 49. PALUDINA. 



37. The tribe of HEMI'CYCLO'STOMA is composed of pectini- 

 branch gasteropods, which resemble snails in the form of their 

 shell, of which the whorl next to the last presents a sunken arch, 

 which gives its aperture more or less resemblance to a crescent. 

 They form many genera, among which we will mention the 

 Paludina, Littorina, Monodonta, Janthina, and Nerita. 



38. The PALUDINJS were, for a long time, confounded with 

 the cyclostomse, but the aperture of their shell is without the 

 rolled lip, and they are provided with branchiae for breathing in 



water ; they show the first vestiges of the 

 syphon \v.hich always exists in the follow- 

 ing family ; we observe on each side of 

 the body a sort of membranous wing, and 

 on the right side this prolongation curves 

 into a small canal, through which water 

 is introduced into the branchial cavity. 

 Many species are found in fresh waters 

 in the United States. A common species, 

 the Palvdina vivi'para, so called because 

 its young are born alive, is marked by 



purplish longitudinal bands, and the shell is greenish ; it abounds 



in the standing waters of France. 



39. The LITTORINA do not differ much from the Paludinse, 

 except that they have a thicker shell, 



and inhabit the sea. The Vigneau, or 

 Turbo littoreusi the shell of which is 

 round, of a brown colour, streaked with 

 black, belongs to this genus ; it abounds 

 on the coast of France, where it is eaten 

 by the natives. 



40. The MONODONS are distinguished 

 from the littorinse by the presence of a 

 blunt, slightly projecting tooth at the base 

 of the columella. The animal generally 

 bears on each side three or" four filaments, 

 as long as the tentacles, and has the eyes 



supported on peculiar pedicles. A small species, the Trochus 

 tesselatvs, with a brown shell, spotted whitish, abounds on the 

 coasts of France. 



Fig. 50. 



LITTORINA VIGNEAU. 



37. What is the characteristic of the Hemi'cyclo'stoma ? 



38. What are the characters of the genus Paludi'na ? (Paludina, formed 

 from the Latin palus, a marsh.) 



39. What are the characters of the genus Littorina ? 



40. What are -the characters of the genus Monodon? 



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