Lamellibranchiata. 333 



wrinkles and crenulations not only vary in number and strength, but 

 in their direction, which is vertical or oblique. There is also a very 

 great difference in the width of the ligamental area. 



Formation and localities. In the shales and shaly sandstones of the 

 Hamilton group, from the eastern limit of the formation as far west 

 as Oanandaigua lake ; it is extremely abundant in Albany and Scho- 

 harie counties ; Oazenovia and Hamilton, in Madison county, and at 

 Pratt's falls, in Onondaga county, N. Y. This species is also found 

 in a cherty limestone in Clarke county, Indiana ; and in similar con- 

 ditions and associations at several localities in Ohio. 



Glyptodesma erectuh, var. obliquum. 



Glyptddesma erectum, var. obliquum. Hall. Pal.N. Y., vol. v, pt. 1. Plates 

 and Explanations : PL 12, fig. 4 ; pi. 13, figs. 5-11. Jan., 1883. 



This variety possesses the essential characters of the typical forms 

 of the species. The hinge, test, etc., are similar, but the body of the 

 shell is distinctly oblique ; the wings are less developed, and the an- 

 terior wing, especially, has not the great extension often observed in 

 the erect forms. The specimens of this variety are uniformly smaller. 

 They have been observed mainly in the soft shales at Pratt's falls, 

 Onondaga county, while the large, erect forms are common in the 

 coarse shales and grits of the group in the eastern portion of the State, 

 and in Madison county,, 



In examining and comparing the extreme forms, the difference is 

 striking and distinctive. But in the study and comparison of a series 

 of several hundred specimens, the intermediate forms appear to unite 

 the whole under a single species. 



To facilitate the examination, the oblique forms are here designated 

 as G. erectum, var. obliquum. 



Glyptodesma occidentale. 



Glyptodesma occidental, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions ; PI. 15, fig. 12. Jan., 1883. 



Shell large, broadly ovate ; body nearly erect ; height and length 

 about equal : margins regularly curved. Left valve very convex ; 

 gibbous on the umbo. Eight valve unknown. Hinge-line straight, 

 equaling or greater than the length of the shell. Beak anterior to 

 the middle of the shell, directed slightly forward, acute and promi- 

 nent. Umbonal region gibbous, defined anteriorly by the broad 



