99 



valve these ridges are more regular in their course ; and on the margins, 

 especially of old shells, they are stronger and more continuous. There is 

 also much variation in the distinctness of the features of the interior, 

 both of the dorsal and ventral valves, the smoother shells being thinner. 

 There is no trace on the interior of the dorsal valve of the ridges on its 

 lateral slopes. 



Size. Length 9 mm. ; width 10 mm. or more. Depth of the two 

 valves together 2 mm. or more. 



Horizon and Locality. In assises d and e of the Upper Etcheminian 

 (E. 3 d and e) Dugald brook, Escasonie, N.S. 



As the outer layer of this shell is thin and fragile, the strong inner Acrothele 

 layer is the one most commonly exposed, and might be thought the real w jth other 

 suiface. The outer surface has an ornate sculpturing, while that of Ihe s P ecies - 

 second layer is smoothed. This species of Acrothele is distinct from all 

 others by the long tubercle or callus in front of the pedicle opening. The 

 ribs of the outer surface of the lateral slopes of the dorsal valve are peculiar 

 to it. A. Matthewi and its varieties have no such ribs. A. Afatthewi- 

 prima has a granulate-latticed surface, but no lateral or anterior ribs. A. 

 Matthewi-costata also has a granulated surface, and ribs on the front of the 

 ventral, but none on the sides of the dorsal valve. The varieties of A. 

 Matthewi also have the foramen nearer the cardinal margin than is the 

 case with this species. 



Examples of this species occurring in the Assise E. 3d differ from 

 those of A. abavia occurring with it, in the thinner corroded valvesj 

 larger size and oblate form ; they are doubtfully referred to this species 

 for the ventral valve is more concave in front of the umbo than are the 

 typical shells occurring in Assise E. 3e. It does not flake at the middle 

 layer of the shell as A. abavia of the same assise does. 



In Assise E. 3d valves appear, which, by their oblate form and surface 

 markings, may, without much doubt, be referred to this species. Not 

 only are they broader than the Acrotheles of the lower assises, but they 

 are larger, some valves attaining 9J mm. in width. An exterior of a 

 ventral which is nearly one-half wider than long, and an interior of a 

 dorsal about a third wider than its length, are figured. 



In the examples from this horizon the ventrals show surface markings, 

 hinge area and foramen ; their interiors show crescentric grooves of the 

 vascular trunks, and at the margin, prints of its branches. Some of the 

 dorsals show the surface sculpture ; others, which have the interior ex- 

 posed, show median and lateral ridges, vascular lines, <kc. 



1 C. K. 



