Class—Pteropoda. 
Head more or less distinct; eyes none; mouth often furnished 
with cup-shaped appendages ; fins on the sides of the mouth or neck ; 
body ovate or roundish, often enclosed in a thin translucent shell. 
Animal free, floating on the surface of the sea ; hermaphrodite. 
ORDER—THECOSOMATA. 
Head indistinct, with two wings on the sides of the mouth. Tooth 
of lingual membrane hooked, with a strong hooked tooth on each side. 
Gills internal. Body inclosed in a shell. 
FAMILY—HYALID. 
Body enclosed in an elongate or globular thin shell. Head not 
distinct ; fins two, large, united. Gills internal. 
Genus, HY ALEA—Lamark. 
Shell globular, translucent ; mouth narrower than the cavity, with a 
lateral slit on each side ; dorsal plate produced into a hood ; posterior 
extremity tridentate. 
H. affinis, D'Or; Cavolina affinis, Gray, Cat. Ptr., Brit. Mus., 
p- 13 Reeve’s Conch. Ic., f. 3 (Pteropoda.) Shell inflated in front, 
flattish behind, broader than high, finely transversely striated on the 
upper anterior portion, almost smooth below and on the _ back; 
posteriorly three spined, the lateral ones short, the middle one slightly 
reflexed ; dorsal lip produced, with three broad rounded ribs, which run 
along the back towards the median spine ; brownish. 
Chatham Islands. 
