CENTRONELLA OF THE ONONDAGA LIMESTONE. 401 



Centrunella alveata. 



PLATE LXI. 



Rhynchonella ? alveata : Hah, Tenth Report on the State Cabinet, p. 124. 1857. 

 Compare Cen(ronc2/a Aerate : BiLLiHOs, Canadian Journal, May 1861. 



Shell elongate-oval or subrhomboidal ; length much greater than the 

 width ; sloping from above the middle to the prominent beak of the 

 ventral valve, the .included angle being about 70°. Anterior margin 

 subtruricate for about one-third the width of the shell, 



Ventk.\l valve prominent in the middle, from a broad, undefined, abruptly 

 or scarcely subangular mesial elevation : the slope thence to the sides 

 below the middle is scarcely convex, and above the middle the sides 

 are more convex, with a little flattened expansion close to the margin ; 

 the longitudinal outline gently arched ; the beak prominent, erect or 

 slightly arching. 



Dorsal valve, in the upper part, convex at the sides and depressed in 

 the middle into a wide undefined sinus, which, below the first third 

 of the length, embraces the entire width of the valve, giving it a 

 trough-shaped aspect. 



Surface smooth : shell compact, punctate. 

 Length of specimen, one inch. 



This species, described by me in 1857, has so much the form and character of 

 C. hecate, Billings, described in 1861, that I can scarcely doubt the identity of 

 the two ; the only difference being in the size of the specimens. 



Geological formation and locality. The original specimen of this species was 

 given to me many years since by Mr. W. H. Pease, as coming from the Onondaga 

 limestone ; but the particular locality is unknown. 



[ Pal^ontologt IV.] 61 



