LAMELLIBKA-NCHIATA 



57 



Class II.— LAMELLIBRANCHIATA. 



(Bivalve Shells.) 



More than a third part of the known fossil shells are 

 ordinary bivalves (Conchifera, Dh.) They amount to nearly 

 (3000, while the recent species scarcely exceed half that 

 number. Nevertheless, it is a group which attains its maxi- 

 mum in the present seas. The genera are seven times more 

 numerous in the newer tertiary than in the oldest geological 

 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, 



Fig. 13. 

 Palaeozoic Bivalves. 



i. Aviculopecten, sp. ; Carboniferous, Belgium. 



2. Posidonomya Becheri ; Carboniferous, Hesse. 



3. Ambonycliia vetusta, Sby. ; Carboniferous, Belgium. 



4. Myalina Goldfussi, Dkr. ; Carboniferous, Vise. 



5. Ctenodonta cuneata, Hall; L. Silurian, Canada. 



6. Lyrodesma plana, Conrad ; L. Silurian, Hudson River. 



7. Axinus obscurus, Sby. ; Magnesian limestone, Durham. 



8. Conocardium armatum, Ph. ; Carboniferous, Tournay. 



9. Pleurophorus costatus, T. Br. ; Magnesian limestone, Durham. 



10. Grammysia cingulata, His. ; Ludlow roclcs, Kendal. 



11. Edmondia, sp. ; Carboniferous, Belgium. 



besides numerous sub-genera. The families Cyprinidce, Astar- 

 tidce, and Anatinidce, have passed their maximum : the 



