SPIRIFER.^ OF THE UPPER HELDBRBERG GROUP. 207 



Figure 23, Plate 31, illustrates the ventral valve of an individual of tliis spe- 

 cies. From the conditions of preservation, it does not represent the full length of 

 the valve in proportion to the width. Mr. Billings has recognized a similar form 

 in Cauada We.st, which he compares with S. miwronata. Our specimens are.very 

 unlike 8. muct-onata. 



Geological formation and locality. In the Upper Helderberg (Corniferous) 

 limestone, near Williamsville, New- York. 



SpirifiBra sc^enta. 



PLATE XXXI. 



Spirifer tt^mextu» : Hau, in Tenth Report on the State Cabinet, p. 131. 1857. 

 " " Description of New Pal. Fossils, p. 91. 1857. 



Shell transverse, semioval ; length less than half the width : hinge-line 

 equalling the greatest width of the shell, and terminating in salient 

 angles. Surface plicate. 



Ventral valve much elevated, subpyramidal, most prominent at the 

 beak, which is not incurved : sinus strongly defined, shallow and 

 nearly flat in the bottom, with the sides straight, giving a triangular 

 form, in which the sides are about once and a half as long as the base. 

 Area very large with sharply angular margins, a little inclined for- 

 ward, and nearly of the same size as the exterior of the valve : the 

 fissure is high and large, being nearly of the same dimensions as the 

 mesial ginus. 



Dorsal valve depressed convex, and flattened towards the cardinal extre- 

 mities, larger than the ventral valve, semielliptical in form, with a 

 low but sharply defined mesial fold which is barely flattened upon the 

 summit. The proportions in height of area, length of dorsal and length 

 of ventral valves, is about as five, six and seven. 



Surface marked by twenty or more simple rounded (or subangular) pli- 

 cations on each side of the mesial fold and sinus, the lateral ones of 

 which do not reach the beak, but run out along the margin of the 

 In its perfect condition the shell has been marked by fine concentric 

 striae, traces of which are still preserved, together with stronger imbri- 

 cating lines of growth. 



