SOME NEW DEVONIC FOSSILS 267 



to margin and number from 16 or less in small individuals to 26 

 in the largest shells seen. In full growth of the shell the median 

 rib becomes more pronounced than the rest on the anterior margin 

 and thus makes a low median angle at this margin. Extremely 

 fine concentric lines are visible under favorable preservation. The 

 cardinal area is narrow and only in rare instances are spines re- 

 tained or developed at the cardinal extremities. On the interior 

 the brachial valve has a small erect V-shaped cardinal process and 

 short thin outer socket walls. On each of the simple coarse ribs 

 which correspond to the furrows of the exterior is a single row of 

 sharp pustules the median row being the most depressed, and the 

 two adjoining the most prominent. No trace of reniform or other 

 lateral depressions is evident. In all such small and coarse ribbed 

 species represented by C. billings i, C. laticosta Hall and 



Chonetes billingsi 



C. m u c r o n a t u s Hall, there is a natural similarity of expression 

 extending even to the interior characters of the brachial valve, but 

 the distinctive differences of C. billingsi consist in its gib- 

 bosity, angulated front margin at maturity, stronger, coarser and 

 more uniformly simple ribs. In respect to these characters in all 

 the shells they are the most pronounced in that before us and be- 

 come progressively decreased in the upward range of the group, 

 so that it serves the purposes not only of paleontologic but also of 

 geographic distinction to recognize distinctively this early manifes- 

 tation of features which are less pronounced in C. laticosta 

 of the Onondaga limestone and still further diminished in C. 

 mucronatus of the Marcellus and Hamilton. 



Dimensions. A full sized example has a width on the hinge 

 of 13 mm. and a length of 11 mm. From this the size ranges 

 downward to a length of 2 mm. 



Lozver Devonic. Grande Greve, P. Q. 



Middle Devonic. Gaspe Basin, P. O. 



