254 PALJUONTOLOGY. 



becoming flattened, or very faintly concave, near the nmge-extremities; 

 hinge-line marked by three or four very short, appressed spines on each 

 side of the beak. Dorsal valve and area of the ventral unknown. Sui'face 

 marked by very fine, closely crowded, radiating striae; the number not 

 determined, owing to the exfoliated condition of the shell. 



The species resembles somewhat C. lUinoisensis Worthen, from the 

 Burlington limestone, in the size and convexity of the valve, and also in the 

 striae, but differs in the greater proportional length of the hinge-line and 

 in the flattening of the mesial portion. 



Formation and locality. — In limestone of the age of the Waverly group, 

 at Logan Canon, Wahsatch Range, Utah. Collected by S. F. Emmons, esq. 



Genus SPIRIFERA Sow. 

 Spirifera centronata. 



Plate IV, figs. 5-6. 



Spirifera centronata Wiuchell, Proc. Acad. Nat. SvA. Phil., 1865, p. 118. 



Shell rather below the medium size, transversely elongate, with mucro- 

 nate extensions, exclusive of which the front margin of the shell forms nearly 

 a semicircle, the length from beak to front being sometimes greater and 

 sometimes less than half the length along the hinge. Valves convex, the 

 ventral most ventricose, with a moderately sized, rather pointed beak, which 

 is slightly incurved and projecting beyond the hinge-line. Area narrow; 

 mesial sinus distinct, narrow, angularly defined at the margins, and extend- 

 ing to the beak; occupied by from three to five plications near the front, 

 formed by the bifurcation of two, which originate at the beak. Sides of the 

 shell marked by from twelve to eighteen or twenty plications, mostly simple, 

 but sometimes bifurcating. The plications are slender and rounded, not 

 very angular. Dorsal valve depressed-convex-,, the narrow fold well defined, 

 but not highly elevated. Minute surface-characters not obsei'ved, as the 

 specimens are all in a state of exfoliation. 



This species seems to be a common form in the rocks near the base of 

 the Wahsatch limestone. It is subject to some variation in external form 

 and number of plications, but is readily recognized and identified. 



Formation and locality. — Near the base of the Wahsatch limestone (Wav- 



