70 



third tlie length from the beak. The beaks appear to be depressed down 

 to a level with the hinge line, but upon this point there jet remains some 

 uncertainty. 



From some nearly perfect casts of the interior, the following characters 

 can be made out. In one of the valves (supposed to be the ventral), a 

 strong rounded groove commences just beneath the beak, and runs along 

 the median line to about the centre of the shell. On each side of the 

 principal groove is a large ovate muscular impression, extending from near 

 the mid-length of the shell a little more than half way to the beak. These 

 impressions are bounded and distinctly defined at their lower extremities 

 by the two small diverging grooves above mentioned. Their outer and 

 upper margins are distinctly defined. In the rostral part of the shell 

 there are two small grooves which take their origin close to the beak, one 

 on each side, and run towards the front, diverging to the outside of the 

 upper part of the two large muscular impressions. The characters of the 

 interior of the dorsal valve are somewhat similar to those of the ventral 

 valve, but the median groove is shorter, and there is a thickening of the 

 shell just below the beak, which presents the appearance of a false area 

 inside of the cavity of the umbo. It is probable that the two small grooves 

 above mentioned are connected with the small muscular impressions, 

 which, in O. chromatica, are distinctly seen outside of the two larger. 

 The condition of our specimens, however, is such, that this point must 

 remain open for further investigation. 



This species difiers from 0. chromatica in the form of the muscular 

 impressions, which are larger and nearer the median line. 0. i^retiosa 

 is of a different form, the width being greater than the length. 0. Ida 

 is much smaller, more convex, and not so nearly circular. 0. nana 

 (Meek and Hayden), Proc. Acad. Nat. Sci., Philadelphia, vol. 13, p. 435, 

 Dec. 1861, is also wider than it is long. 



The two large muscular impressions in this species appear to converge 

 towards the front in some of the specimens, as in the genus Obolus, while 

 in 0. cliromatica they diverge. But, notwithstanding this difference, the 

 laro-e size of tliese scars, and the general aspect of the shell, appear to be 

 sufiicient to authorise us to place it in Oholella. 



Locality and Formation. — Point Ldvis ; in the graptolitic slates, 

 Quebec group. 



Collectors.— '^vc W. E. Logan, J. Richardson. 



