No. 89.] 81 



Obthisina arctostbiata. 



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



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

 del tidium. 



ORTHISTNA ALTERNATA ( n.s.). 



Shell of medium size, semielliptical : hioge-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. 



Obthisina altebnata. 

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



AMBOCCELIA GREGARIA ( n.s.). 



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

 Spirifer unguiculus, Phillips, Pal. Fossils, p^. 28, f. 119. 

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



f. 6 a, 6, 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 ^. umbonata in the less r^jgular convexity of the ventral 



valve, the greater convexity of the dorsal valvefiynjd 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 conyiiiced me that it is quitq 



distinct, 



[ Senate No, 89.] 11 



