60 PALEONTOLOGY 



and some other oolitic species, have two ranges of little hinge- 

 teeth, but not like those of the recent species of Limcea. The 

 large and smooth or striated Lamas of the oolites have been 

 called Plagiostoma, a name originally given by Llhwyd. 



The pearl-oysters (Aviculidce) are also very abundant 

 fossils : but owing to the frecpient repetition of similar forms, 

 it is difficult to determine the genera with any degree of cer- 

 tainty by the aid of external characters alone. The Silurian 

 species mostly belong to the genus Pterinca (Goldfuss), and 

 are broadly winged, and have the hinge-area striated length- 

 wise, and a few diverging hinge-teeth. Amboncychia (Hall) 

 resembles Inoceramus, and ranges from the Silurian to the 

 carboniferous strata (fig. 13, 3). The Silurian genus Cardiola 

 is ridged like a cockle ; and Posidonomya, which is found in 

 all the palaeozoic rocks, is very thin and concentrically fur- 

 rowed (fig. 13, 2). Many other genera have been proposed whose 

 characters are even more imperfectly understood. Monotis 

 (Salinarius), one of the common shells of the trias, has no 

 anterior ear. Pteroperna (Lycett), an oolitic form, has a 

 winged shell, with numerous small anterior teeth and long- 

 posterior laminae. The genus Gervillia (fig. 14, 4), ranging 

 from the carboniferous limestone to the chalk, consists of elon- 

 gated shells, with several cartilage-pits in the ligamental area. 

 Bakcwellia, found in the Permian, has an anterior muscular 

 impression like Area. The recent genus Pcma commenced 

 in the lias or preceding formation, and exhibits great variety 

 of shape. Pulvinites Adansonii (fig. 14, 3) appears to have 

 been a Perna with a byssal foramen like Anomia ; ami Inoce- 

 ramus (fig. 11, 5), characteristic of the cretaceous strata and 

 oolites, differs from Perna chiefly in form, the larger valve 

 being sometimes completely involute, and resembling a 

 Nautilus. The genus Pinna, which appears to belong to this 

 family, although provided with two adductor muscles, occurs 

 fossil in the Devonian and all subsequenl strata. Some of the 



