MOLLUSCA. 77 



compose the surface or exterior crust of our globe than the remains of 

 elephants, hippopotami, and whales." 



A''iewiiig this group as a class, D'Orbigny divides it into six orders, each 

 of which, excepting the first, is subdivided into two families. Of the recent 

 species, 68 genera and 900 species are known, more than half of which 

 belong to the warm seas. D'Orbigny's opinion of their classification is as 

 follows : Though less complicated in their organization than many zoophyta, 

 they have not a common life, nor are they aggregated like them, whilst 

 their locomotive powers and testaceous envelope place them much higher. 

 On the other hand they are in all respects less complete than the echino- 

 dermata ; and judging from the radiation of their filaments, the position of 

 the foraminifera is in Cuvier's radiated division, between the Echinodermata 

 and the Zoophyta, as a distinct class. 



Okder 2. Pteropoda. This order takes its name from two large 

 expansions, one on each side anteriorly, which are analogous to the wings 

 of a butterfly both in appearance and action, being used in locomotion. 

 This order was considered by Lamarck and Cuvier to form a distinct class, 

 but Blainville and other distinguished naturalists are of opinion that its 

 contents are essentially gasteropoda. Some species, as Clio horealis (pi. 76, 

 fig. 78, one inch long), are naked ; others, as Uyalma., are provided w'ith a 

 delicate shell. These molluscs are monoicous, and swim about in the sea 

 without the power of creeping or attaching themselves to solid bodies. 

 This order contains the families Hijalmdce., CUdidcn, and PliiHirJididm. 

 In Hyalsea the branchiae are composed of pectiniform transverse laminae or 

 leaves^* a character which would aftbrd a name for tlie order in consonance 

 with those of the allied orders, because Cuvier's name, Heteropoda^ apper- 

 tains to the group as a class. 



Fam. 1. ByaloeidcE. The shell of Hyal<Ta has a delicate texture and 

 curious form, bearing a distant resemblance to that of Terebratula, to 

 which the genus was at one time referred. The shell which covers the 

 abdomen is translucent, subglobular, and has the aj^pearance of a bivalve in 

 which the two valves unite in a single piece without a hinge. The mollusc 

 has two stout tentacles and two lips, and the wing-like expansions are placed 

 near the mouth. They live in the high seas, seldom approaching the shores, 

 flapping themselves along with great facility, and descending when dis- 

 turbed. Being extremely abundant where they occur, they are devoured 

 in great numbers by whales and other animals. Cleodora and Limacina 

 are other genera. 



Fam. 2. CUdidce. Clio has six retractile tentacles in two groups, and a 

 rudimentary foot. Clio borealis is abundant in the north polar regions, 

 where it is eaten by whales and fishes. 



Fam. 3. PMllirhcädiB. The head of Phylliroe is lengthened, and bears 

 two tentacles, the snout is retractile, and the body is gelatinous and trans- 

 parent, very much compressed laterally, and provided with a caudal fin. 

 According to Peron and Lesueur the branchiae are internal, and in the form 



* The group which Latreille named Phyllohranchia, had already been provided with a 

 name. 



281 



