94 



Variety 

 crassa. 



ACROTHYRA PROAVIA-CRASSA, PL II, figs, 5 tt-C. 



Bull. Nat. Hist. Soc. of N. B. vol. iv, p. 389, figs. 5 a-c. 



Only the ventral valve known. This is short, tumid and conical, 

 Interior. This possesses a narrow callus, four or five times as long as 

 wide, and nearly a third of the length of the shell. At the front of the 

 callus are two small oval scars divided by a faint septum. The callus is 

 concave and extends back nearly to the beak. 



Sculpture. Some fragments of the surface which are preserved show 

 fine, close set, concentric ridges. 



Size. Length, 2J mm. ; width, 2 mm. ; height, 1J mm.? 



Horizon and locality. Lower layers of the assise E. 3 e, at 

 Dugald brook, Escasonie (C. B.), N. S, Also a doubtful ventral from 

 E. 3 /. at Gillis, Indian brook, Escasonie. Scarce. 



This mutation is distinguished from the type and from mutation 

 prima by its robust form, and from proavia, the type, also by the posses- 

 sion of a thickened callus. From the mutations and type of A. signata by 

 the narrowness of its callus. 



Of the two species of Acrothyra herein described, signata was found 

 specially to characterize the lower half of the Lower Etcheminian fauna, 

 being found most abundant in the middle measures of this set of beds. 

 It is not, however, limited to these measures, but by mutations is 

 sparingly represented in the upper part of this lower fauna. 



Acrothyra proavia, on the contrary, has been found only in the Upper 

 Fauna, and mostly in its higher part, where some layers are crowded 

 with thousands of these litte shells. 



Acrotreta 



comparative 



abundance. 



ACROTRETA. Kutorga. 



While this genus appears as, a contemporary of Acrothyra in the 

 earliest Basal Cambrian, it seemingly lived on after the latter had passed 

 away. But throughout the Coldbrook and lower Etcheminian measures, 

 it is quite subordinate in numbers to Acrothyra, and we have not found 

 it at all in the upper Etcheminian. Throughout the true Cambrian in 

 the Acadian Provinces, however, these conditions were reversed, for, 

 with the doubtful exception of Lingulella (Acrothyra ?) inftata of the 

 Protolenus fauna, an undoubted example of the genus Acrothyra is 

 unknown to me above the Etcheminian horizon, and Acrotreta has full 

 possession of the field in the higher Cambrian zones. 



