68 PALAEONTOLOGY. 



carboniferous limestone {e.g., P. Sowerbyi) cannot be 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 (fig. 17, i), and their hinge- 

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

 The most beautiful forms occur in the chalk and greensand, 

 and resemble the recent scallop (Janira, Schum.) in the 

 inequality of their valves, but are further characterized by the 

 possession of articulating hinge-teeth like Spondylus. These 

 constitute the genus Neithea (fig. 18, *). Plicatulce exist in 

 the trias and oolites, along with shells referred dubiously to 

 Hinnites and Sj^idylus. True Hinnites (a sub-genus of 

 Pecten) are characteristic of the miocene. Spondyli appear in 

 the greensand and chalk. Some of them (like the so-called 

 " Plagiostoma spinosum") are unattached ; others resemble 

 the recent deep-water 8. Gussonii, and have been called 

 " Dianchorse." The inner layer, including the hinge of these 

 shells, is seldom preserved. Lima proboscidea first appears 

 in the lower oolite, and reappears in the great oolite, and in 

 the Kelloway rock. Lima duplicata, 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 Limas of the oolites have been called Plagiostoma, 

 a name originally given by Ilhwyd. 



The pearl-oysters (Aviculidce) are also very abundant 

 fossils : but owing to the frequent 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 Pterinea (Goldfuss,) and 

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

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

 resembled lnoceramus, and ranges from the Silurian to the 

 carboniferous strata (fig. 17, 3). The silurian genus Cardiola 

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



