142 



class. Their habitat is the sea, on whose surface they swim wth the back 

 downwards. The Firolidw have large bodies, either altogether unprotected, or 

 with a small glassy shell attached ; they live in warm seas. The Atlantidce 

 have large shells, into which they can withdraw themselves at pleasure, and close 

 them wth an operculum. Maclurea is a Lower Silurian genus, of which 

 Murchison gives four species. BeUerophon has eighteen species dispersed among 

 all the Silurian formations : the species figured is not uncommon at Dormington 

 Wood. BeUerophon dilatatus is a giant species; BeUerophon trilobatus a, dvfaxt 

 of the genus. 



Fuller particulars about the Gasteropoda, and the strata, of which the 

 several species are characteristic, with many figures, may be found in the new 

 edition of Siluria. 



CLASS PTEROPODA. 



(Thecosomata.) I (Gymnosomata. ) 



*Hyaleidse. Cliidse. 



Limaoinidse. | 



This small class, the lowest of univalve encephalous molluscs, represents 

 in this sub-kingdom, the Birds of the Vertebrata and the Insects of the 

 Articulata : they propel themselves through the water, to which sphere of 

 existence they are altogether limited, by means of a fin-like expansion on each 

 side of the head and neck, furnished with muscular fibres, and they derive their 

 name from the resemblance to winc/s of these organs of motion. Their internal 

 structure is very complex, presenting many points of resemblance both to the 

 Cephalopoda and Gasteropoda, and yet of a type distinct enough to entitle them 

 to the rank of a separate class. They possess in most cases minute eyes, 

 projecting prehensile tentacles, and lingual teeth, adapting them, frail as they 

 are, to prey upon the still frailer Crustacea of the ocean. They form two sections, 

 one consisting of the families whose members have a case for their bodies 

 (Thecosomata), the other of the bare-bodied families (Gymnosomata) ; the latter 

 section would not be likely to leave behind it in sub-marine deposits any traces 

 of its existence ; myriads of its best known genus Clio are seen swimming in the 

 high latitudes, where they provide the whales with their chief means of sub- 

 sistence. Shells of the family Hyaleidce, distinguished by their glassy delicacy, 

 are found in the miocene beds of Italy ; but about the occurrence of Pteropodous 

 shells in the older rocks there is much uncertainty. The only paloeozoic genera 

 placed by Woodward in this class are Theca, Pterotheca and Conularia, the two 

 former being exclusively Silurian, and the latter ranging thence to the Car- 

 boniferous formation : Murchison with some doubt adds Ecculiomplialus : 

 Owen is inclined to connect with these the genus Euomphalus and the Lower 

 Silurian Maclurea, of which we have already spoken in the Gasteropoda. The 

 new edition of Siluria gives twenty-seven species in this class, fourteen 

 belonging to the genua Theca, which has, however, only two representatives in 



