visceral callus, and most numerous on the ventral valve. The dorsal 

 valve shows three grooves at an earlier stage than the ventral, but in the 

 latter the grooves are more numerous and are inost abundant toward the 

 anterior margin ; there are perhaps one-half more of these grooves on 

 the ventral than on the dorsal valve ; for often there is only one, and 

 rarely more than two on this, but frequently three on the ventral. The 

 following table shows the relation of these grooves to the length of the 

 shell. 



VALVES of L. Selwyni from Assise E. 1 (a 1) 

 (The dimensions given are in millimetres. ) 



Dimensions of 

 L. Selwyui. 



Sculpture. The umbonal region in this species is granulated. Out- 

 side of this region the granulation soon becomes varied by concentric 

 striae. In the anterior half of the valve, strong concentric furrows of 

 growth, still further diversify the surface. 



