70 



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

 to a level with the hinge line, but upon this point there yet 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 

 strono- rounded groove commences just beneath the beak, and runs along 

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

 principal "roovc is a large ovate muscular impression, extending from near 

 the mid-leno'th of the shell a little more than half ^vaj to the beak. These 

 impressions are bounded and distinctly defined at their lower extremities 

 by the t^YO small diverging gnwves aljove mentioned. Their outer and 

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

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

 on each side and run towards the front, di\'crging to the outside of the 

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

 interior of the dorsal valve are somewhat simihir 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 proliablc that the two small grooves 

 above mentioned are coiniected with the small muscular impressions, 

 which in 0. chromatlca, 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 differs from 0. cJiivimdica in the foi-m of the muscular 

 impressions, which are larger and nearer the median line. 0. pretiosa 

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

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

 (Meek and Hayden),Proc. Acad. Nat. Sei., Philadelphia, vol. 13, p. -loS, 

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



The two larr'e muscular impressions in this species appear to converge 

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

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

 laro-e size of these scars, and the general asjiect of the shell, appear to be 

 sufi&cient to authorise us to place it in Ol>olclla. 



Localitii and Formation. • — Point Levis ; in the graptolitic slates, 

 Quebec group. 



Collectors.— ^yji: W. E. Logan, J. Richardson. 



