AMBOCCELIA OF THE CHEMUNG GROUP. s 261 



Aniboccelia umbonata, var. gregaria. 



PLATE XLIV. 



Orthis unguiculus : (Phillips sp.); Hall, Geol. Report Fourth District, pp. 267, 268. 1843. 

 Jtmbocalia gregaria : Hall, Thirteenth Report on the State Cabinet, p. 81. 1860. 



The shell occurring in the Chemung group, and indicated by me as a 

 distinct species of this genus, possesses so many characteis in common 

 with the Hamilton forms that I hesitate to continue the specific designa- 

 tion. 



The ventral valve is often more elongated, and the margins of the 

 mesial sinus are subangular. The dorsal valve is depressed convex, with 

 a distinct median sinus as shown in the casts. The casts of the interior 

 of the ventral valve show a longitudinal median crest in the muscular 

 impression, which is also often limited at the sides by a linear obtuse 

 impressed line, as if by some callosity on the interior of the shell. 



Some of the specimens in the Chemung group attain larger dimensions 



than any of those in the Hamilton, being often nearly half an inch in 



diameter. 



This form, -vvith its convex dorsal valve, bears much resemblance to the Sjpinfera 

 urii* of Europe, which, according to Mr. Davidson, passes from the Devonian 

 through the Carboniferous, and occurs in the Permian system. It is described by 

 Mr. Davidson as having a narrow mesial furrow in the dorsal valve, which is 

 " nearly flat or slightly convex". This author remarks : " The external surface, 

 where perfect, is covered with small spinules". Although the shell of this fossil in 

 the Hamilton group is apparently in a very perfect condition, I have been quite 

 unable to discover any evidence of the existence of spinules or their bases, a 

 character observed in Spirifers allied to 8. lineata. 



Geological formation and localities. This variety occurs in the shales and shaly 

 sandstones of the Chemung group, near Binghamton, at Chemung and Chcmung- 

 narrows; near Elmira, at Corning, Paintedpost, and along the Corning and Bloss- 

 burgh railroad; at Jasper in Steuben county; near Olean in Allegany county; at 

 Randolph, Albion, and Connewango in Cattaraugus county; Gerrytown-corners, 

 Cherry-creek, Ellington and other places in Chautauqua county. 



*Mrypa unguiculu>,BowERBT = Spirifera unguicu/o, Phillips, is regarded as asynonymof 5. unV. 



