ASIPHONIDA. 



487 



ever, are inflated and incurved, the hinge-line is curved, and 

 a row of transverse teeth exists on each side. Mytilarca, 

 distributed from the Upper Silurian to the Carboniferous, 

 possesses a mytiloid shell, with anterior beaks ; a striated 

 ligamental area being present, with lateral and cardinal teeth 

 placed along a curved hinge-line. Lyrodesma (Actinodonta), 

 of the Lower Silurian, is. an extremely well-marked type, 

 with an equivalve oblique shell, truncated behind, and with 



Fig. 349. Cyrtodonta Hindi. (Billings.) Lower Silurian, a, Side view ; 6, Dorsal view. 



a hinge of several transverse cardinal teeth radiating in a 

 fan-shaped manner from the beak (fig. 348, A). 



The shells of the genus Cyrtodonta 1 (Palcearca of Hall) are 



1 So much controversy has been carried on as to the Palaeozoic shells known 

 by the names of Cyrtodonta, Palcearca, Megalomus, Megambonia, and Cypri- 

 cardites, that it would be impossible to discuss here which of the groups so 

 named can be finally retained. So far as the author can judge, Cyrtodonta of 

 Billings has priority over Palcearca of Hall, and the latter must be abandoned. 

 Megalomus may be usefully retained as at least a sub-genus. Megambonia, of 

 the Upper Silurian and Devonian, has affinities with the Aviculidce, but may 

 be really referable to Cyrtodonta. Lastly, Cypricardites appears to include a 

 number of diverse types, and probably the best course would be to discard 

 this name altogether. 



