AMBOCCELIA OF THE CHEMUNG GROUP. 
2®1 
Amboctelia umbonata, var. gregaria. 
PLATE XLIV. 
Orthis unguiculus : ( Phillips sp.); Hall, Geol. Report Fourth District, pp. 267, 268. 1843. 
Ambocalia 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, with its convex dorsal valve, bears much resemblance to the Spirifera 
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 
chai*acter observed in Spirifers allied to S. lineata. 
Geological formation and localities. This variety occurs in the shales and shaly 
sandstones of the Chemung group, near Binghamton, at Chemung and Chemung- 
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. 
*Atnjpa unguiculus, Sowerby == Spirifera unguicula, Phillips, is regarded as a synonym of N. urii. 
