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. 
Preroropa. Six genera. 
Some genera of this family are without a testaceous covering, 
mentioned only to preserve the family entire. 
1. Genus Hyaleza. Pl. 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. 
‘T'wo species. 
Hyalea tridentata. Hyalea cuspidata. 
2. Genus Clio. Pl. IX. 
Animal. Body free, naked, more or less elongated, tapering to 
the rear, head very distinct; provided with six, long, conical, re- 
tractile tentacule, 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. 
