74 



callus is of a circular form, and only one quarter of the length of the 

 valves. Its ridge closely encircles a deep pit, which lies just in front of 

 the foraminal opening, in the direction of which it becomes narrower and 

 shallower. The traces of a pair of straight diverging grooves are discern- 

 able at the sides of the callus. 



Sculpture, This consists of minute concentric ridges, visible only with 

 a strong lens. 



Size. Length, 2mm.; breadth, 2^ mm.; depth, 1 mm. 



Horizon and locality. Fine gray shales in the volcanic beds at Dugald 

 Brook, Escasonie (C.B.), N. S. Scarce. 



The short callus distinguishes this species from Acrothyra signata-prima 

 with which it occurs. The pit in this callus, though so short, is analog- 

 ous to that of A. signata, so that in this earliest fauna these two types of 

 umbonal muscle scar and groove of the Acrotretinze were already differ- 

 entiated. 



LEPTOBOLUS TORRENTIS, N. SP. PL. vi, FIG. 1. 



Leptobplus Shell thin, surface shining. Form elongate-oval, 



torrentis. 



Ventral valve, obtusely pointed at the back somewhat acutely rounded 



in front ; elevated along the middle ; somewhat flattened along the sides 

 within the margin. 



Interior. By decortication a low boss and a transverse furrow behind 

 it are exposed on the mould, about three quarters of the length of the 

 valve from the hinge ; if this boss marks the front of the callus, the 

 - central muscles are unusually far forward, more advanced even than in 

 L. atavus of the Etcheminian terrane. There are faint impressions of the 

 lateral septa on the sides of the valve. 



Dorsal valve unknown. 



Sculpture. The surface of the shell is covered with minute, low tuber- 

 cles, cancellate in arrangement ; through these can be traced faint parallel 

 lines, concentric to the uinbo. Along the median third in places can be 

 seen about six broad flat ridges, radiating from the direction of the umbo ; 

 these break the continuity of the concentric ridges. 



Si ze , Length, 3mm ; width, 2 mm ; depth, mm. 



Horizon and locality. Gray shales in the volcanic rocks of the Cold- 

 brook terrane. Scarce. 



