RHYNCHONELLIDJE OF THE CHEMUNG GROUP. 
363 
Ventral valve usually moderately convex, gibbous on the umbo, gently 
curving or flattened on the sides, depressed in the middle into a wide 
shallow sinus; apex closely incurved. 
Dorsal valve convex, more or less gibbous, with the mesial fold promi¬ 
nent below the middle of the shell. 
Surface of mesial fold and sinus marked by four, five or six well-defined 
rounded plications, which reach nearly or entirely to the apex, and 
are dichotomous in their upper part. Sides of the shell smooth, or 
obscurely marked by low obsolete folds; concentrically marked by 
fine striae. 
In the larger number of specimens, the lateral portions of the fossil 
are essentially free from plications, but they are sometimes visible, and 
in a single small specimen there are two plications on either side of the 
sinus; and one larger specimen shows a similar character. In others, 
there are inequalities of surface, indicating the plications. It should be 
recollected, however, that the specimens are usually in the condition of 
partial or entire casts, and these features may have been more strongly 
developed in the shell. 
The cast of the dorsal valve presents the usual features of all the spe¬ 
cies in the longitudinal septum, which is distinctly divided on its upper 
inner face. 
It is not difficult to conceive how, by a gradual obsolescence of the lateral pli¬ 
cations of the L. qucidricostata of the Genesee slate, and by acquiring a more 
robust habit in a different sediment, this form in the Chemung group may be 
only a modification of that which holds a position in strata several hundred feet 
below. Although retaining the specific designation for the prevailing Chemung 
form, I am quite prepared to see the propriety of uniting it with the species 
just named, and perhaps even with the L. multicosta of the Hamilton group. 
Geological formation and localities. This species occurs in numerous localities 
of the Chemung group in the central part of the State. It has been collected near 
Ithaca, at Chemung and Chemung narrows, near Elmira, Bath, etc. I have not 
seen the species in the western counties of Cattaraugus and Chautauque. 
