No. 3.] BILLINGS — ON THE GENUS OBOLELLINA. 327 



Genus Obolellina, Billings. 



Generic characters. — Shell, unarticulated, ovate or orbicular, 

 smooth or concentrically striated. Area of the ventral valve with 

 a median groove, on each side of which there is, sometimes, an 

 additional furrow. In the interior of this valve there are two 

 large, ovate, or sub-rhomboidal muscular impressions. They are 

 situated near the centre, but usually (for the greater part) in the 

 posterior half of the shell. They are sometimes obliquely striated 

 or grooved, or obscurely reticulated by both transverse and 

 longitudinal striae. Close under the area there is a fine, but dis- 

 tinctly impressed groove, which curves outwards and forwards, 

 outside of the muscular scars for a greater or less distance towards 

 the front margin. There appears to be an enlargement of this 

 groove, just under the peduncular groove of the area, on the 

 median line, as if for the attachment of a muscle. The large 

 scars are bordered anteriorly, by an elevated margin, which is 

 prolonged forwards, along the median line, in a more or less pro- 

 minent ridge ; this ridge varies greatly in the amount of its deve- 

 lopement, in different individuals of the same species, being some- 

 times almost obsolete. 



The area of the dorsal valve varies greatly in size in the diffe- 

 rent species, and is either flat or with a triangular elevation under 

 the beak like a pseudo-deltidium. Beneath the area there is a 

 fine groove, which curves outwards and forwards, as in the ventral 

 valve, with a similar enlargement in front of the beak. There 

 are two large, ovate, sub-central muscular impressions, with a 

 smaller pair in front of, or between them. These latter are 

 situated on or close to the median line, and usually appear as a 

 single scar, but in some specimens are distinctly divided into two, 

 by a longitudinal ridge. Their form varies in different indivi- 

 duals of the same species. The muscular impressions are margined, 

 anteriorly^ by an elevated border, which is extended forwards as 

 an obscure rid<2;e, a e;reater or less distance towards the front. 



In the original notice it is stated, that this genus has no cavi. 

 ties in either valve. This holds good for all the specimens of 0. 

 Canadensis of which the interior has been seen. In 0. Galtensis, 

 however, while some of the specimens have no cavities, in others, 

 as is shown by the casts of the interior, there is a small one extend- 

 ino: a short distance under the larger muscular scar on each side in 

 the ventral valve. In one of our specimens there is a short cone, 

 half-a-line in length, on the edge of the cast of the cavity. No 



