Genus Obolella. (N. gen.) 



Generic characters. — Shell ovate circular or sub-quadrate, convex or 

 plano-convex. Ventral valve with a false area which is sometimes 

 minute and usually grooved for the passage of the peduncle. Dorsal 

 valve either with or without an area. Muscular impressions in the ventral 

 valve four ; one pair in front of the beak near the middle or in the upper 

 half of the shell, and the others situated one on each side near the car- 

 dinal edge. Shell calcareous. Surface concentrically striated, sometimes 

 with thin extended lamellose ridges. 



In general form these shells somewhat resemble Oholus, but the arrange- 

 ment of the muscular impressions is different. In Oholus the two central 

 scars have their smaller extremities directed downwards, and converging 

 towards each other ; but in this genus the arrangement is exactly the 

 reverse. 



The three species in which I have seen the muscular impressions are 

 the following: — 



1. — 0. chromatica, — hereinafter described. In this species the central 

 muscular impressions are divergent below. 



2. — A species which occurs in the well-known deposit of limestone near 

 Troy in the State of New York. This is probably Avicida ? desquamata. 

 (Hall) (Pal. N. Y., vol. I, p. 292, plate Ixxx, fig. 3.) In two specimens of 

 this species which I have before me the scars are in the upper part of 

 the valve and diverge below. The small scar on each side close to the 

 margin is visible. 



3. — A small species from the Potsdam sandstone of the St. Croix 

 River in the Western States, where it occurs associated with the Primor- 

 dial trilobites described by the late eminent geologist. Dale Owen. In 

 this the central scars are close together, one on each side of the median 

 line and parallel. 



The genus appears to be closely alhed to Obolus, but sufficiently dif- 

 ferent, on account of the disposition of the muscular impressions, to be 

 classified as distuict therefrom. 



Obolella chromatica. (N. sp.) 



Fig. 1. 



Fig. 1. Obolella chromatica. — a. Ventral valve ; 6. dorsal ; d. interior of one of the 

 valves, supposed to be the ventral, showing the muscular impressions ; c. outline 

 restored from detached valves. 



