LAMELLIBEANCHIATA. 



67 



stone, after which the species become abundant, and are 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, 





Fig. 17. 

 Palceozoic Bivalves. 



i. Aviculopecten, sp. ; Carboniferous, Belgium. 



2. Posidonomya Becheri ; Carboniferous, Hesse. 



3. Ambonychia 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. Grammy sia cingulata, His. ; Ludlow rocks, Kendal. 



11. Edmondia, sp. ; Carboniferous, Belgium. 



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

 the upper valve, is someway within the margin of the shell. 

 And Garolia (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 Pectinidce are very numerous. Some of them in the 



