ORTHIDES OF THE HAMILTON GROUP. 51 



or interrupted, from the peculiar manner in which the pores open upon 

 the surface. 



Interior of the ventral valve marked by a subcircular or broadly ovate 

 flabellate muscular impression, which occupies more than half the 

 length and breadth of the valve, and, in old specimens, is extremely 

 thickened from its anterior margin nearly to the border of the palleal 

 impression ; the ovarian spaces distinctly pitted, pustulose or rugose, 

 in curving ridges. Dental lamellae strong and prominent, merging below 

 into the raised border of the muscular impression. 



Interior of the dorsal valve showing a strong prominent cardinal process, 

 which is continued in a median ridge sometimes nearly to the front of 

 the shell. In some specimens, low transverse ridges separate the ad- 

 ductor muscular scars ; while in other specimens, and particularly the 

 older ones, this division is scarcely distinct, and the muscular impres- 

 sion is broadly oval, with its lower margins showing foliate impressions 

 as in the muscular impression of the ventral valve. 



This species reaches a larger size than 0. vanuxemi; often measuring more than 

 one inch and three quarters in transverse diameter, while the largest specimens 

 of O. vanuxemi seldom measure more than one inch. It differs also in the character 

 and strength of the radiating strias ; the muscular imprint in the ventral valve is 

 usually broader and more strongly marked ; the cardinal and brachial processes 

 of the dorsal valve are stronger, and directed towards the opposite valve; while 

 these, in 0. vanuxemi, are inclined forwards, or into the cavity of the shell. 



In specimens of the larger and medium size, the ventral valve is quite flat or 

 concave below the middle. The tubular character of the stria? is more strongly 

 developed than in 0. vanuxemi or 0. leucosia; while in the finer punctate texture 

 of the shell, there is no distinction perceptible. In all the authenticated specimens 

 of this species, the cardinal line of the dorsal valve is straigiiter, and the beaks 

 not approximating. The strong similarity between the three species above named 

 will render it difiicult in many cases to determine them. 



Geological formation and locality. In the calcareous shales of the Hamilton 

 group, at Eighteen-mile creek and Hamburgh, Erie county ; Alexander and 

 Pavilion, Genesee county; York and Moscow, Livingston county; and on the 

 shores of Canandaigua and Seneca lakes. 



