96 



is somewhat squared at the sides ; the groove outside of the callus is 

 somewhat indistinct. On each side of the foramen is sometimes a short, 

 sharp furrow directed forward. In the dorsal valve the depressed posterior 

 lateral slopes and the somewhat flattened anterior slope give the valve 

 a triangular appearance. The interior shows a pit at the hinge area, 

 which is narrow, and thence a narrow median ridge runs nearly to the 

 middle of the valve. An inconspicuous lateral branch is thrown off each 

 side of the median ridge. 



Horizon and locality. The assise E. 1 d, on Boundary brook, Esca- 

 sonie, (C.B.) N.S. 



ACROTItETA. sp. 



A species of this genus occurs in the sandstone of E 2a at Youngs 

 point with Lingulella Selwyni. It is rare, and only a dorsal valve has 

 been found. 



DEVELOPMENT OP ACROTRETA. 



Development This is one of the most conservative of the genera of the Cambrian and 

 3 a ' Ordovician. Though its species occur at intervals at various horizons in 

 these Systems the uniformity of size and sculpturing is remarkable. Though 

 so uniform as regards their outer surf ace, the various species of Acrotreta 

 present differences of moulding of the interior of the valves, especially of 

 the ventral valve by which they may be distinguished. This may be seen 

 by consulting the figures of some of the Canadian Cambrian species given 

 on plates iii, iv and xviii, where especially in the moulds of the ventral 

 valves marked differences of form of the inside surface is apparent. As re- 

 gards the moulds of the interiors of the dorsals a^o, the differences are suffi- 

 ciently marked. (Compare with each other, figures 2^/, 4c and 5h of plate 

 iii, 2g of plate iv, and fig. 2 of plate xviii.) The ornamentation consists 

 of a fine concentric striation, only visible with a strong lens. The 

 size did not increase more than about four-fold in area in the vast 

 space of time included in the Cambrian and Ordovician Systems. Con- 

 trast this with Paradoxides, which increased in area an hundred fold in 

 the first two sub-faunas of the Paradoxides Zone. These sub-faunas per- 

 haps do not cover more than a twentieth of the space of gelogical time 

 represented by the range of faunas in the following table. 

 The series of Acrotreta run in size about as follows : 



