LAMELLIBRANCHIATA. 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 

 inequi valve. 



Entire surface marked by fine, crowded, concentric striae, 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 Iv, 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. 



