134 



the middle, or with a median depression, and convex at the 

 sides. 



The character of the genus, as given in the descriptions 

 and illustrations of Mr. Billings, are as follows: 



GENUS CENTRONELL A , Billings, 1 1859. 

 " Generic characters : Shells, having the general form of 

 Terebratula. Dorsal valve with a loop consisting of 

 two delicate riband-like lamellae, which extend about 

 one-half the length. These lamellae at first curve gently 

 outwards, and then approach each other gradually, until 

 at their lower extremities they meet at an acute angle ; 

 then becoming united, they are reflected backwards 

 towards the beak in what appears to be a thin flat vert- 

 ical plate. Near their origin, each bears upon the ventral 

 side a single triangular crural process. Name from the 

 Greek, Kentron, a spur. This genus is intermediate be- 

 tween Terebratula and Waldheimia. In the former the 

 loop is short, not exceeding greatly one-third the length 

 of the shell, and not reflected. In the latter, it extends 

 nearly to the front, and is reflected, but the laminae are 

 not united until after they are folded back." 



Fig. 13 (4). Interior of the dorsal valve, showing the 

 loop. 



1 Description and figures copied from the Canadian Naturalist and Geolo- 

 gist for April, 1859 : the figures enlarged three diameters. 



Fig. 13, 



Fig. 14. 



