91 



This valve is often undulate with one, sometimes several strong grooves 

 concentric to the umbo, marking periods of rest in the growth of the 

 shell ; corresponding ridges are found on the deltidial area. 



The dorsal valve is round, and broadly rounded in front ; the contour 

 of the surface is varied by a moderate projection of the umbo behind, and 

 by a slight flattening of the valve-in front, giving the valve a rounded, 

 slightly triangular relief. The interior has the impression of a pair of 

 muscles in the umbo, whence a low ridge extends forward across the 

 valve. Not infrequently the edges of this valve are flattened, and one 

 or more grqoves, concentric to the umbo, marking stages of growth, 

 indent it. 



Sculpture. A strong lens reveals a series of concentric striae on the 

 surface on some examples of this shell, there being about 20 in the space 

 of a millemetre. Between these ridges a still stronger magnifier (1 inch 

 objective) shows a fine granulated surface with occasional rows of coarser 

 granules, parallel to the concentric striae. On the inner, chitinous surface 

 there is a similar ornamentation, but less distinct than that on the sur- 

 face of the outer layer. 



Size. Length of the ventral valve in the largest examples, 3 mm. ; 

 width, 2 mm. The dorsal valves in both diameters is 2 mm. Depth of 

 the ventral valve from the beak 1 \ mm. ; that of the dorsal at the middle, 

 \ mm. A great majority of the valves are smaller and of the size given 

 in the original description. 



Horizon and locality. In the Assise e, common (and less common in 

 d where it is larger) of the upper Etcheminian, Dugald Brook, Escasonie, 

 Cape Breton. Very thin shells are found on the highway at V. McPhee's 

 in Assise e. 



There is a good deal of variation in the form of the ventral valves of this 

 species. The majority are of the dimensions given, but sometimes the 

 width of the valve is equal to the longest diameter. Also the concentric 

 furrows of growth are in some examples so profound as to give the ventral 

 valve, when slightly distorted obliquely, the appearance of a minute 

 Raphistoma. 



This species differs from all others of the Acrotretihse known to me, except Attitude of 

 L. inflata of the Protolenus fauna, in the high overhanging apex, which the 

 in the typical form projects one-quarter beyond the base of the valve, but 

 in one variety from Assise e, one-third beyond. As a result of their form, 

 the ventral valves of this species, in place of standing erect like many of 

 those of the genus Acrotreta, rest on the dorsal side, on the layers of the 

 shale in which they are imbedded, and except for their marked convexity 



