58 PALAEONTOLOGY 



Trigoniaclce are nearly extinct ; and the Hijopuritidce 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 (Afyacidce and Ana- 

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

 Veneridcc 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 Qryphoea 

 (fig. 14, i ) abounds in the oolites, and is gregarious, but 

 unattached, the umbo of the larger valve being curved inward 

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

 stone, after which the species become abundant, and arc with 

 difficulty distinguishable from the smooth and plaited, or 

 " cocks-comb," oyster of the present day. 



Several curious modifications of Anomia and Placuna 

 have been obtained in a fossil state. Limanomia (Bouchard) 

 has ears like Lima, and is attached to shells and corals of the 

 Devonian age. Placunopsis (M. and L.), found in the oolites, 

 has a transverse ligamental groove, which, like the umbo of 

 the upper valve, is some w T ay within the margin of the shell. 

 And Carolia (Cantr.), a tertiary form of Placuna, has a byssal 

 plug passing through a foramen like that of Anomia when 

 young, but closed in the adult, 



Fossil Pectinida: are very numerous. Some of them in the 

 carboniferous limestone (e.g., P. Sowerbyi) cannot bo distin- 

 guished generically from the living Pectens, and retain diverg- 

 ing bands of colour. But the greater part of these old species 

 are somewhat aviculoid in form (tig. 13, >), and their hinge- 

 area is grooved with cartilage-furrows, like those of Area. 



