LAMELLIBRANCHIATA. 155 



of the species. In the young shell the posterior wing is proportionally more 

 extended than in older individuals, while the anterior wing is usuall}' smaller. 

 During the progress of growth, both wings often become extravagantly 

 developed, and the form and proportions of these parts cannot be relied upon 

 for specific distinctions. The characteristic forms have the body nearly erect, 

 with the umbo and beak directed slightly forward ; but associated with these, 

 are other forms which present a considerable degree of obliquity in the body 

 of the shell, and while there seem to be no constant marks of specific 

 distinction, it is extremely unsatisfactory to group them all under one species. 



The interior characters also present considerable differences, the number of 

 lateral teeth varying from one to three ; the cardinal 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 ligamehtal 

 area. 



Formation and localities. In tlie shales and slialy sandstones of the Hamilton 

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

 lake ; it is extremely abundant in Albany and Schoharie counties ; Cazenovia 

 and Hamilton, in Madison countv, 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 conditions and associations at several locali- 

 ties in Ohio. 



Glyptodesma erectum, var. obliquum. 



■PLATE Xir. FIG. i; and PLATE XIII, FIGS. o-II. 



Glyptodesma erectum, va.v. obliquum. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanation.s ; PL 12, tig. 



4 ; 1)1. 13, fig-s. ."i-U. 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 anterior wing, especially, has not 

 the great extension often observed in the erect forms. 



