NIAGARA GKOUP. 



263 



ceding, has its representative in the shaly limestone of the Helderberg, where a larger species 

 presents the same characters of surface in all its details. 



Fig. 3 a, b, c. Ventral views, presenting the ordinary characters of well preser\'ed specimens. 



Fig. 3 d. An individual, still preserving its natural proportions, where the beaks are widely se- 

 parated, with a broad cardinal area and little extension of the cardinal extremities. 



Fig. 3 e, f, g. Profile views of three specimens, showing tlie variable elevation of the beak of 

 the dorsal valve, and width of the area. 



Fig. 3 h, i. Front views of specimens, showing a different proportional convexity. 



Fig. 3 k. A portion of the surface enlarged. 



Position and locality. In the shale at Lockport, Lewiston, and rarely in other places. 



624. 7. SPIRIFER BICOSTATUS. 



Pl. LIV. Fig. 4 o - c. 

 OHhis bicostatus*. Vanuxem, Geo!. Rep. 3d Dist. N. York, 1842, pag. 91 and 94. 



Somewhat obovate-triangular, the dorsal valve gibbous, with the beak extended and incurved 

 over a short triangular area ; ventral valve convex ; surface marked by conspicuous concentric 

 subimbricating striae ; dorsal valve with a distinct plication on each side of the sinus, and 

 towards the base are two other obscure plications on each side, presenting three and sometimes 

 four gentle undulations on the tnargin on each side of the centre ; cardinal line shorter than 

 the width of the shell, and the area scarcely extending so far as the cardinal line ; extremities 

 distinctly rounded. 



This species is intermediate in its characters between S. crispus and S. ptychodes. The con- 

 centric striae are slightly dissimilar from those of S. crispus, in appearing to imbricate more 

 distinctly. The plications, except one on each side of the sinus of the dorsal valve, rarely reach 

 the beak, and usually appear only near the base or on the margin. It may be readily confounded 

 with S. crispus ; but the shorter hinge-line, and abrupt curving of the striae at the extremities, 

 are sufficient to distinguish it from that species, even where the plications are insufficient. 



The specimen figured has the ventral valve compressed, and I have not been able to see one 

 with this valve perfect, though the dorsal valve is not uncommon. 



Fig. 4 a. Dorsal valve of an individual of this species. 



Fig. 4 h. Ventral view of the same, showing the area and foramen, the beak of the ventral valve 



being broken off. 

 Fig. 4 c. Profile view, showing the form of the dorsal valve. 

 Fig. 4 d. Front view, in which the elevation of the ventral valve is given as it appears to have 



been in its perfect state. 

 Fig. 4 e. An enlarged portion of the surface, showing the character of the strias. 



• Tliis species was labelled by Mr. Conk ad, in Mr. Vanuxem's collection, Orthis bicostnlus ; but the name has 

 been published, so far as I know, only in the report of Mr. Vanuxem cited above. 



