118 



PROF. 1\ M. DUNCAN ON THE 



but possibly more inclined than is usual in that genus. The 

 hollow for the reception of the umbo of the opposed arm-bone is 

 deep and long from above downwards, and it occupies all the 

 median line as far down as the upper edge of the notch of the 

 inferior canal. There is no "peg." On either side of the hollow 

 for the umbo is a raised, rounded ridge extending from the 

 apophysis downwards and on either side to reach the outer part 

 of the upper edge of the lower muscle-area. On each rounded 

 ridge is a nodule situated far above the notch for the lower canal 

 (PI. XI. fig. 23), so that a nodule occurs on either side of the 

 median hollow for the umbo, and it is placed above the origin of 

 the downward projecting roots of the canal-notch. The nodules 

 are slightly elongate and project ab orally, and are articulating 

 surfaces. Beyond the nodules, on the side remote from the 

 median line, are slight depressions. 



The lower canal is large, and the edges of the lower muscle- 

 area are raised. 



On the corresponding adoral surface of an arm-bone (fig. 24), 

 the upper process, or slot for the reception of the apophysis of 

 the adorally placed bone, is large. Below is a long umbo, rounded, 

 broad and projecting in the upper part, narrow and with a 

 median projecting narrow ridge midway, and tumid, broad and 

 projecting at the lowest part, which is at the upper edge of the 

 comparatively small lower canal. This long umbo is therefore 

 most tumid, largest, and most projecting at its upper and lower 

 extremities, and is narrow and less projecting midway. 



There are no knobs on this surface, and the only approach to 

 any such articulating process is a pair of indefinite enlargements 

 where the upper edge of the lower muscle-area unites with the 

 descending raised ridge of the sides of the lower canal. But the 

 depressions for the nodules of the opposed aboral surface of the 

 next bone are seen on the sides of the umbo close above its lower 

 enlargement. 



The absence of a peg or its analogue, the length of the hollow 

 for the umbo, and the presence of a pair of nodules characterize 

 this species in reference to the aboral surface of the arm-bone. 

 On the other hand, the long umbo and the defective knobs of the 

 adoral surface are just as peculiar*. 



The Jarge size o\ the slot and the obliquity of the apophysis 



* The description is taken from jointl ncur tlie dink ; much modification 

 occurs fur out in the urm. 



