800 Lamarck's Genera of Shells. 



the belly or neck. One or more fins, not disposed in pairs, or 

 regular order. 



First Order. 



Pteuoi-oda. 



Most of the Pteropoda are small animals, with no appen- 

 dices, or only very short ones on the head. Some have a thin 

 cartilaginous or horny shell, and some have branchial fins. 



1st Family, 

 Hyalteana. (6 Genera.) 



1. Hyalasa *. 



Body covered with a shell ; two opposite, rather large, re- 

 tractile fins inserted at each side of the mouth. Scarcely any 

 head. Mouth terminal, situated at the junction of the fins. 

 No eyes. Branchiae lateral. Shell horny, transparent, ovate- 

 globular, posteriorly tridentate, open at the summit and two 

 posterior sides. 



The shell of the Hyalaea appears, according to Forskahl, to 

 consist of two valves cemented together. The valves are un- 

 equal; the largest, dorsal, rather flattened below, the other 

 ventral, tumid, subglobular, and shortened anteriorly. The 

 middle one of the three posterior teeth, or points, is perforated. 

 On each side of the shell, is a very open fissure, to admit the 

 water to the branchia;. 



Type. Hyalcea t7-identataf. (Monoculus telemus ? £ihh.) 



Shell yellowish, pellucid, thin, very delicately striated trans- 

 versely ; terminal point longer than the lateral. 



Mediterranean, 2 Species. PI, VII. Fig. lOGj. 



^ From iiaXoc, glass. f Having three teeth. 



X We have fjiven a figure of anotlicr, and, we believe, liitlierfo nnptib- 

 lisliei), genus, which seems to belong to this family. It was collected by 

 the late Mr. Cranch, on the Cong;o Kxpetlition, and piTseiited to the Bri- 

 tish Museum, ("where it is preserved, with another specie";, apparently of 

 the same genus,) Vjy the Lords (lomnii^sioncrs of the Admiralty. We 

 propose to call it, at the suggestion of a kind and learned friend, Bulan- 

 tiMn reciirvum *. As the animal inhabitant, however, is quite unknown 1o 

 us, we place it in this family, merely from the strong analogy which the 



* From liaXavIitiv, a purse ; recurvum, recMat'ed— the apex being bent. 



