40 



slopes ; beaks central, incurved, moderately elevated ; pos- 

 terior extremity obliquely subtruncate ; anterior end appa- 

 rently rounded ; base nearly straight, and parallel to the 

 cardinal edge ; elevated part of the posterior muscular at- 

 tachment not very prominent. Surface of cast retaining 

 faint remains of radiating costag, and lines of growth. 

 Length 1*73 inches ; height '92 inch ; breadth *80 inch. 



Although I have seen but internal casts of this species, I 

 have ventured to characterize it, believing the central posi- 

 tion of its beaks, straight basal margin, and sub-angular 

 posterior umbonial slopes, will be sufficient to distinguish 

 it. There is in the collection, from the same rock, cast^ of 

 a species having an ovate form, with the beaks nearer the 

 anterior end. 



Arca Yaxcouverexsis. — Shell oblong, rather gibbous in 

 the middle ; anterior end rounding up from below, angular 

 at the end of the hinge line above ; posterior end vertically 

 truncated ; base nearly straight, and parallel with the hinge. 

 Beaks a little in advance of the middle, rather small, not 

 much elevated ; posterior umbonial slopes angular from the 

 beaks obliquely backwards to the lower part of anal ex- 

 tremity ; ligament area straight, and narrow. Surface or- 

 namented by small radiating costal, and much finer closely 

 arranged concentric lines; on the anterior end about every 

 fourth one of the costas is larger than those between, while 

 further back they are often alternately larger and smaller. 



Length *74 inch ; height *44 inch ; breadth *40 inch. 



This species will be readily distinguished by its vertically 

 truncated posterior extremity, and the distantly separated 

 radiating costa), with smaller ones between, on the anterior 

 end. I know of no species with which it is liable to be con- 

 founded. 



Cardium scitulum. — Shell very small, circular or sub- 

 quadrangular, gibbous in the middle ; rounded in front, and 



