PTEROPODA. 



209 



Order I. 



THECOSOMATA. 



The body of the animal is most generally enclosed in a 

 shell; the head not distinct; the fins, a single pair; no 

 outward gills. 



Family l.— CA VOLINIA CEA. 



Animal with the body large, perfectly symmetrical, with 

 the gills in pairs. Shell calcareous, symmetrical, elongate 

 or globular, and possessing no operculum. 



Cavolinia. Gioeni. (Tricla Retzius. Hyalasa Lam. 

 Archonta Montf.). — Shell horny or 

 glassy, transparent, ovate, globular, 

 tridentated posteriorly, open at the 

 summit and at the two posterior sides. Hyateatrkpinosa. 

 Animal, with a globular body having two wing-like 

 lateral expansions, which are projected from the slits at 

 the sides of the shell, and a broad expanded two-winged 

 natatory disk extending on each side of the anterior 

 extremity; head not distinct; branchiae lodged in a 

 special cavity ; no operculum.* — About 20 species ; also 

 fossil. 



These curious little shells are found in the Mediter- 

 ranean and Atlantic. C. trispinosa has for its trivial 

 name in England that of Venus' Chariot. 



Clio. Lin. (Cleodora Peron and Lesueur). 

 — Shell cartilaginous, transparent, in the shape 

 of a reversed pyramid, truncated, and open at 

 the summit. Animal, lengthened, without side 

 appendages to the mantle ; two distinct tenta- 

 3ula ; the under apron-formed prolongation of the fin (or 



* Forbes's British Moll. 

 P 



