122 MANUAL OF NATURAL HISTORY. 



merous in species ; their shells are varied in form, 

 and are glassy, clear, and transparent ; they are 

 beautiful objects, but on account of their fragile 

 nature and their inaccessible oceanic habitats, are 

 not often found in the Collections of Conchologists, 

 or even in those of National Museums. 



Many peculiar forms of Pteropods seem to swarm 

 in the polar seas ; and one genus, the Limacina, 

 occurs in such prodigious multitudes as even to 

 constitute the principal food of the mighty Baleen- 

 Whale. The long pallial prolongations often pro- 

 truded from the fissures in the sides of the shells, do 

 not appear to be of much service in propelling or 

 guiding these animals through the water, but are 

 most probably subservient to respiration. 



II. CLASS. PTEROPODS (Pteropoda). 



Head more or less distinct ; eyes none ; mouth 

 often furnished with cup-like appendages ; fins two, 

 usually on sides of mouth, often with an interme- 

 diate lobe ; body ovate or roundish ; unisexual ? 

 free, floating on the surface of the sea by means of 

 their fins ; crepuscular or nocturnal. 



I. OKDER SHELL-BEARING PTEROPODS (Thecosomata). 



Head indistinct, with two wings on sides of mouth; 

 tooth of lingual membrane hooked, with a strong 

 hooked tooth on each side ; gills internal ; body en- 

 closed in a shelL 



1. FAMILY. Spiny-Pteropods (Cavolinidse). Fins 

 united, no foot-like appendage between them ; 



