PTEROPODA, 241 



few instances the shell becomes modified into a kind of 

 gelatinous cartilaginous integument. 



Pte}'02)oda are separated into two separate families by 

 De Blainville, namely G^mnosomata and Thecosomata, naked 

 and conchiferous. As, however, the latter only pertains to 

 the science of shells, they are now divided at once into six 

 genera, as follows : — Hyalaa, Cleodorcij SjoirateUa, Creseis, 

 Ciwieria, Cijmhulia. 



The habits of these singular mollusks have been already 

 noticed ; and as regards that interesting species the Hijalcea, 

 Professor Scouler speaks of having met with it on a mass of 

 floating algae which he captured when at sea. He often 

 amused himself wdth observing the agility with which the 

 active little being climbed up the branches by means of a 

 grooved foot, and that frequently with his back undermost, 

 resembling in this respect the sloth, which inhabits the vast 

 forests of South America, and whose natural liistory has 

 been only of late understood. 



As regards the shell of tlie Pteropods, we may briefly 

 mention, that similarity, and yet diversity, is conspicuous in 

 each. The shell of the Ilj/alcea is globose and reddish- 

 brown on the uppermost part ; that of the Cleodora is tri- 

 angular, thinner, and consequently more brittle and trans- 



