SPIKIFEK^ OF THE UPPER HELDERBERG GROUP. 201 



The casts show considerable variation in the form and proportions of the mus- 

 cular area, two examples of which are shown in figures 15 imd 16, Plate 29. 



The cast of a ventral valve, Plate 34, figure 25, illustrates the character of a 

 well-presei-ved specimen from the Hamilton group. 



This species has been recognised by Dr. F. Rcemer and M. de Verneuil as 

 . identical w^ith the S. cultrijugata of Europe. Without the means of comparing spe- 

 cimens, I had supposed the American and European species to be identical ; 

 but Mr. Davidson has recently called-attention to the existence of bifurcating ribs 

 in the American species, while " in all the figures published by Rcemek, Schnub, 

 " and Sandbergek of 8. ctiltrijitgatus, the ribs on the lateral portions of the shell 

 " are simple, and do not exceed about twenty on each valve ; but in a specimen so 

 " named in my possession from the Falls of the Ohio near Louisvillej^ in America, 

 " and which I received likewise some jears ago from Herr F. Rcemek, the ril>s are 

 " small, and in some places bifurcated, and numbering about twenty -four in each 

 " valve." 



The original figures' of Dr. Rcemer represent the species with simple strong 

 rounded ribs on each side of the mesial fold and sinus ; while the elevation in 

 front is triangular as in our specimens. In the figures of Schnub,* the sinus in 

 front is not triangular, but the sides approach to parallel, and the summit is 

 arched rather than angular. It appears to me, however, that M. Schnur has indi- 

 cated something like a bifurcation, or a depression along the centre of the larger 

 plications, in figures 1 a, i ; though it must be confessed that the general expres- 

 sion of the figures is somewhat different from our shell. The figures given by Mr. 

 Davidson (loc. cit.), and referred with doubt to this species, have " six or seven 

 " strong simple ribs on each of the hiteral portions of the valve." This feature 

 would preclude its association with our specimens, and indicates a nearer approach 

 to some of the forms which I have referred to 8. macrothyris. 



Mr. CoNKAD has given the length of 8. acuminatus as 2^ inches. The specimens 

 before me vary from one inch to two and a quarter inches in width, and from 

 three-fourths of an inch to an inch and seven-eighths in length. 



Geological formations and localities. This species is not known below the Cor- 

 niferous limestone, where it has been found in Albany and Schoharie counties, as 

 well as at several localities in the central part of the State, and at Williamsville 

 and Clarence-liollow in the western part of the State. It occurs in the same forma- 

 tion at Sandusky and Columl)US, Ohio ; at the Falls of the Ohio both in Louisville 

 and Jefft'rsonville, and at Camp creek, Indiana. It is found in the Hamilton group 



• PalsBontographica, T. iii, pi. xxxiii, f. 1, a, b, c. 

 [ PaLjKontoi.ooy IV.] 26 



