156 (^Senate 



I have ssen this species only in the form of casts, or with a portion of 

 the shell adhering. It resembles S. fornacula, from the same geological 

 position ; but it is a much larger shell, with fewer plications and a more 

 shallow and less angular sinus. The area of the present species is propor- 

 tionally less elevated. In general form it bears a resemblance to S. macro- 

 nota, but the plications arc stronger and fewer than in that species,- and 

 the area is proportionally higher and more arcuate. 



Geological positiofi and locality. Limestones of the age of the Hamil- 

 ton group : Illinois. 



Spirifer wortheni ( n. s.). 



Pal. N.Y. Vol. iv. 



Shell transversely oval, very gibbous, length more than two-thirds 

 the width ; hinge equalling the greatest breadth of the shell ; 

 extremities salient : dorsal valve semicircular, gibbous, distinct- 

 ly arcuate longitudinally, rising in the middle into a very pro- 

 minent rounded fold ; beak and area incurved : ventral valve 

 very convex in the region of the umbo, having a deep rounded 

 sinus extending from beak to front, where it terminates in an 

 obtuse triangular projection ; beak arched, and projecting be- 

 yond that of the other valve; area high, strongly arcuate, trans- 

 versely and longitudinally striate ; foramen large, forming an 

 equilateral triangle : about sixteen or seventeen simple rounded 

 or subangular plications on each side of the mesial fold and 

 sinus. Surface marked by fine radiating strise, which are crossed 

 by undulating lines of growth. 



This fine Spirifer seems to be intermediate in form between iS. acumi- 

 nata, Conrad ( S. cultrijugatus, Romer ), and S. granulifera of the 

 Hamilton group, but diflfers from both by obvious characters. The area is 

 much wider, and the hinge extremities more salient, than in S. acuminata, 

 while the mesial fold is somewhat less elevated and much less acute ; and 

 the plications are likewise simple. 



This species is more angular in outline, the valves less rotund and more 

 unequal in size, the area larger, and the sinus and fold more angular than 

 in S. granulifera. 



Geological position and locality. In limestone of the age of the Ha- 

 milton group : Calhoun county, Illinois. From Mr. A. H. Worthen. 



