186 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 

 1 2 



Orthisina arctostriata. 



Fig. 1. Ventral view of a medium-sized specimen. 



Fig. 2. Enlarged cardinal view of another specimen, showing the closed rounded pseudo- 

 • deltidium. 



ORTHISINA ALTERNATA(ii.s.). 



Shell of medium size, semielliptical : hinge-line shorter than the greatest width 

 of the shell; cardinal extremities rounded. Ventral valve most gibbous near the 

 umbo and depressed near the front : area moderate, somewhat arcuate; pseudo- 

 deltidium large, broad at base, imperforate, marked along the middle by a 

 deeply impressed line. Dorsal valve regularly convex, apparently without sinus; 

 area linear or obsolete. Surface marked by fine radiating striae, alternating in 

 size, usually three smaller between the larger ones near the margin of the shell; 

 distinctly undulating concentric striae. The margin of the shell is extremely 

 thin. 

 This species differs from the last in the proportionally shorter hinge-line and 



the alternating larger and smaller striae, which are also less prominent and less 



closely arranged. 



Geological formation and locality. Shales of the Hamilton group : Genesee 



county, N.York. 



1 2 



Orthisina alternata. 

 Fig. 1. Dorsal valve of a small individual. Fig. 2. Dorsal valve of a larger individual. 



AMBOCCELIA GREGARIA ( n. s.). 



Compare with Atrypa unguiculus, Sowerby, Geol. Transactions, Vol. v, pi. 54, f. 8. 

 Spirifer unguiculus, Phillips, Pal. Fossils, pi. 28, f. 119. 

 Orthis unguiculus, Hall, Geol. Report Fourth District of New-York, p. 268, 



f. 5 a, b, c, d ; p. 267. 



Shell subhemispherical, wider than long : hinge-line straight; cardinal angles 

 rounded. Ventral valve gibbous, marked by a shallow mesial sinus, which ex- 

 tends from near the beak to the base of the shell : beak obtuse, strongly incurved. 

 Dorsal valve semielliptical, depressed convex, with sometimes a slight longitudi- 

 nal central depression ; foveal plates slender and parallel. 

 This species differs from A.umhonata in the less regular convexity of the ventral 

 valve, the greater convexity of the dorsal valve, and the proportionally greater 

 transverse diameter. 



I had originally considered this shell as identical with Atrypa unguicula (So- 

 werby, Spirifer unguiculus (Phillips), placing it under the Genus Orthis; 

 but farther comparison of figures and descriptions has convinced me that it is quite 

 distinct. 



