LAMELLIBRANCHIA TA . 
361 
marked by several radiating nodose ridges with intermediate sulci, which, with 
the exception of a single strong ridge and furrow, become obsolete before 
reaching the middle of the shell, while the remaining furrow and ridge 
extend to the base and constrict the margin. The ridge is developed in the 
right valve, and carries the majority of the nodose radii anterior to it. The 
deepest furrow is developed in the left valve, and carries the principal part 
of the nodose radii posterior to it. This feature makes the shell somewhat 
inequivalve. 
Entire surface marked by fine, crowded, concentric strim, which at some¬ 
what regular intervals are fasciculate and elevated into strong angular ridges 
or undulations. In the region of the cincture these are crossed and inter¬ 
rupted by radiating ridges extending from the beak, and are more 
developed in the young shells, and become obsolete below the middle in 
old shells. Surface also marked by fine radiating pustulose striae, as shown 
in fig. 11 of plate lv, but which are more continuous than represented in the 
figure. Anterior muscular impression sub-circular, situated just within the 
anterior border of the shell. 
Two specimens measure respectively 50 and 80 mm. in length, and 32 and 
55 mm. in height. Several large individuals have lengths of about 100 
mm. each. 
This species resembles G. bisulcata, and has generally been confounded with 
it, but is more oblique, with a shorter hinge-line; the lunule in front of the 
beaks is smaller; the concentric folds finer and more numerous, besides being 
angular, lamellose and fasciculate, with a greater number of interstitial ones. 
The most conspicuous difference is in the oblique folds of the umbo, which 
become nodose at their intersection with the concentric folds. 
The typical forms of this species are very easily distinguished, but in the 
older shells the characteristic features often become obsolete, and the shells 
being distorted by pressure are not so easily separable from other species. 
Formation and localities. In the shales of the Hamilton group, near Sherburne, 
Chenango county; Hamilton, in Madison county; Pratt’s falls, Onondaga 
county, and on the shores of Cayuga lake, N. Y. 
