88 



the front margin. The margin is somewhat straightened at the hinge, 

 and there is a depressed pseudodeltidium, with a narrow striate area on 

 each side. The area is at right angles to the base of the valve, which, 

 when viewed from the side, has the margin somewhat arched up at the 

 front and back. Interior. This has near the hinge a thick rectangular 

 callus, hollowed at the middle, with a depression that deepens toward the 

 hinge ; from this it is divided by a low transverse ridge, behind which 

 is a pit leading to the foramen, which is just behind the umbo. On each 

 side of the callus, two low ridges extend forward at a wide angle, and 

 limit the area occupied by the lateral muscle scars. The position of the 

 central group of muscles in front of the callus is not clearly defined. 



The dorsal valve is orbicular and the umbo depressed. Two broad 

 obscure ridges radi^e from the umbo to the sides of the valve. When 

 viewed sidewise the valve is seen to be bent down both at the anterior 

 and posterior ends. Interior. The most prominent feature is the median 

 septum, which is usually visible from one- sixth of the length of the valve 

 from the back, to the middle of the valve. A pair of diverging grooves 

 originate at the hinge line, and forward, towards the sides of the valve, 

 divide off the space occupied by the impression made by the lateral 

 muscles. Midway between these grooves and the median septum, are 

 two faint vascular ridges. The margins of both valves are thickened and 

 flattened. 



Sculpture. This consists of fine concentric ridges with smooth intervals 

 between ; the known surface is smooth and shining, but there are frag- 

 ments of what appears to be an outer layer, with a dull, minutely granu- 

 lated surface. The surface is often ridged with growth lines, especially 

 toward the anterior margin. 



Size. Length and width of the valves equal, 2^ mm. The depth of 

 the ventral is 1^ mm. ; that of the dorsal, 1 mm. 



Horizon and locality. The Assise Ic of the Etcheminian at Dugald 

 brook, Escasonie (C.B.) N.S. 



This differs from the type in the shorter and wider shell, upright hinge 



A characteris- area > wider visceral callus, and straighter back of the ventral valve. 



tic Etchemi- From A. signata prima in the more regularly conical form of the ventral 



valve. This genus is particularly Etcheminian, there being two species and 



several varieties or mutations in the strata of this age. It seems likely 



Linyulella (?) injlata of the Protolenus Fauna belongs to Acrothyra ; if so, 



the genus ranges up into the base of the St. John terrane. 



