ANCIENT GENERA OF LAMELLIBRANCHIATA. 497 



Mollusca, several of the existing genera date back to the Primary 

 period. An attempt is sometimes made to separate the Primary from 

 the Secondary species of the same genus, and although this is often 

 legitimate from the point of view of classification, we prefer in this 

 summary to use generic names in the large synthetic sense of the 

 older naturalists. 



The Primary rocks are characterised by the extinct genera of bi- 

 valves, Pterincea, Ambonychia, Modiolopsis, Lyrodesma, Cardiomorpha, 

 Cardiola, Conocardium, Megalodon, Anthracosia, Eurydesma, Pachy- 

 dornus, Poseidonomya, Myalina, Sol&niya. A few genera range into 

 the Secondary Rocks, like Poseidonomya, which reaches the Trias. 

 Pleurophorus and Axinus also reach the Trias. 



Ostrea is said to commence in the Carboniferous rocks, but 

 abounds in the Secondary and all newer strata. 



Among the genera characteristic of the Secondary rocks are 

 Inoceramus, which ranges to the Chalk and may begin in the Silurian ; 

 Aucella, which begins in the Permian and ranges to the Gault ; Myo- 

 roncha commences in the Permian and ranges up to the Middle 

 Tertiary. Monotis and Myophoria are Triassic. Sphcera ranges from 

 the Trias to the Lower Tertiary. Opis is limited to the Secondary 

 Strata, as are Exogyra, and Gryphcea. Cardinia, Goniomya, and 

 Vnicardium extend through the Lias and Oolites. Tancredia charac- 

 terises the Lias and Lower Oolites. Hippopodium is Liassic. Ptero- 

 perna, Macrodon, and Pachyrisma are distinctive of the Lower Oolites. 

 Protocardium ranges from the Lower Oolites to the Chalk ; Ceromya 



Fig. 123. Ostrea. Fig. 124. Protocardium. Fig. 125. Iiioceramus. 



and Cercomya range from the Lower Oolites to the Upper Greensand. 

 Discerns and Isodonta are distinctive of the Middle Oolites. Mono- 

 pleura and Requinia range from the Neocomian to the Upper Green- 

 sand. These genera have no recent representatives. 



The living genera which may be considered to have representa- 

 tive species that date from the Primary rocks are Avicula, Area, 

 Cypricardia, Lucina, Cardium, Pinna, Pecten ; Mytilus does not 

 appear till the Permian. With the Trias Lima, Plicatula, Pema, 

 Nucula, Trigonia, Cyprina, Isocardia, Cardita, and Corbis come in. 

 Among genera which appear with the Lias are Astarte, Mactra, 

 Pholodomya, Teredo, Leda. The Lower Oolites first make us ac- 

 quainted with Anomia, Litlwdomus, Limopsis, Venus, Tellina, Cor- 

 bula, Panopea, Anatina, Thracia, and Gastrochwna. There are no 

 new genera introduced in the Middle or Upper part of the Oolites, 



VOL. I. 21 



