LAMELLIBRANCHIA TA. 
155 
of the species. In the young shell the posterior wing is proportionally more 
extended than in older individuals, while the anterior wing is usually 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 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 Canandaigua 
lake; it is extremely abundant in Albany and Schoharie counties; Cazenovia 
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 conditions and associations at several locali¬ 
ties in Ohio. 
Glyptodesma erectum, var. obliquum. 
PLATE XII, FIG. 4; AND PLATE XHI, FIGS. 6-11. ‘ 
Glyptodesma erectum, var. obliquum, Hall. Pal. N. Y.. vol. v, pt. 1. Plates and Explanations : PI. 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 anterior wing, especially, has not 
the great extension often observed in the erect forms. 
