RHYNCHONELLID.E 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. quadricostata 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 

 holow. 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. 



