06 PALEONTOLOGY. 



such species derive extrinsic shelter by burrowing in sand or 

 stone. The mantle is that portion of the skin of the Lamelli- 

 branch which, after investing the viscera, gills, and foot, is 

 reflected, ventrad, in the form of plates and " lobes" (fig. 14, ab) 

 to line the shell which it has formed, and be produced, when 

 needful, into breathing-tubes. 



More than a third part of the known fossil shells are 

 the ordinary bivalves of the leaf-gilled class* They amount 

 to nearly 6000, while the recent species scarcely exceed 

 half that number. Nevertheless it is a group which attains 

 its maximum in the present seas. The genera are seven times 

 more numerous in the newer tertiary than in the oldest geo- 

 logical system ; and the number of species found in the entire- 

 Silurian series is less than 100, while the chalk contains 500, 

 and the miocene 800. Out of 150 genera,. 35 have become 

 extinct, besides numerous sub-genera. The families Cyigri- 

 nidce, Astartidce and Anatinidce, have passed their maximum; 

 the Trigoniadce are nearly extinct ; and the Hippiiritidce have 

 no living representatives. 



The monomyary bivalves, and others with an open mantle, 

 attain a degree of importance at an early period ; and with 

 them some of the burrowing families (Myacidce and Ana- 

 tinidce) ; while the highest organized siphonated shells {e.g., 

 Veneridce and Tellinidce), unknown in the older rocks, are 

 most abundant now. 



The family Ostreidce, distinguished from the Pectens and 

 Anomice by resting on the left valve, contains two fossil forms. 

 Of these, Exogyra resembles an oyster with spiral umbones, 

 directed backward, or to the left hand ; it is an attached shell, 

 characteristic of the cretaceous strata. The genus Gryphcea 

 (fig. 18, i) abounds in the oolites, and is gregarious, but 

 unattached, the umbo of the larger valve being curved inward 

 like a claw. A single Ostrea occurs in the carboniferous lime- 



* Acephales Testaees, Cuv. Conchifera of Lamarck and Deshayes. 



