86 CONCHOLOGY. 



possessing, at their summit, arms disposed in the form of a coro- 

 net : their mouth either short, elongated, or tubular, exertile, and 

 generally armed with hard parts. Mantle diversified, having its 

 edges free on the sides of the body, or the lobes united, forming a 

 sack, which in part envelopes the animal ; gills or respiratory 

 organs various, circulation double, one particular, the other gen- 

 eral ; heart unilocular, sometimes with the auricles divided, and 

 very distant ; no medullary cord along the body, but scattered 

 nerves and ganglions. Twenty-two families. 



FAMILY I. 



Ptekopoda. Six genera. 



Some genera of this family are without a testaceous covering, 

 mentioned only to preserve the family entire. 



1. Genus Hyalaea. PI. IX. 



Animal. Body enclosed in a shell, winged before, two opposite 

 wings, somewhat retractile, inserted at the sides of the mouth, 

 head distinct, mouth terminal, placed at the junction of the fins ; 

 without eyes. 



Shell. Symmetrical, very thin and transparent, valves unequal, 

 flat above, convex below, open like a cleft anteriorly, summit 

 truncated and tridentated posteriorly. Inhabits the Mediterranean. 

 Two species. 



Hyalsea tridentata. Hyalaea cuspidata. 



2. Genus Clio. PI. IX. 



Animal. Body free, naked, more or less elongated, tapering to 

 the rear, head very distinct ; provided with six, long, conical, re- 

 tractile tentaculffi, separated into two groups of three each ; mouth 

 altogether terminal and vertical, eyes sessile, rudiment of a foot 

 under the neck. 



This is a molluscous animal without any testaceous covering, 

 but is here placed as leading to other genera which have shells. 

 Inhabits the Indian seas. Two species. 



Clio borealis. Clio Australis. 



