ADDITIONAL NOTES ON THE CAMBRIAN OF CAPE BRETON 385 
The dorsal valve has not been separated from that of Aerotreta 
papillata, which occurs with it. 
Sculpture. — This, on the lateral slopes of the valve, consists of fine, 
closely vset ridges, visible only with a lens. 
Size. — Length, 2| mm.; width, 3 mm.; depth, 1| mm. 
Horizon and locality. — In the gray shales of E. 1 c and d at Dugald 
Brook, Escasonie, (C. B.) N. S. Common in the latter assise. 
This mutation is distinguished from Acrotreta papillata, with which 
it is associated, by the form of the callus, etc., and from Acrothyra 
signata (typical) by its flatter callus and deeper and shorter lateral 
groves; the same characters distinguish it from A. signata-prima and 
A. signata-sera. 
Acrothyra signata-orta. PI. XIII, figs. 4 a-f. 
This rather tumid form has an overhanging beak. Ventral valve 
broadly ovate, bluntly pointed, convex along the back, especially 
toward the front of the valve, where the curve becomes abrupt. 
Interior — A callus about three times as long as its width in front, ex- 
tending from the beak one-third of the length of the valve, sometimes 
there is an apophysis in front of it, of equal width, sometimes an ap- 
parent extension of the callus, with a median ridge dividing it length- 
wise. The callus usually has a transverse raised thread towards the 
posterior end, and sometimes another near the front. A faint, nar- 
rowly triangular hollow, divides the callus from the impression of the 
lateral muscles. 
The dorsal valve is oblately orbicular in form, with inconspicuous 
umbo. The valve is somewhat depressed in the middle and toward 
the front. Interior — This part of the valve exhibits a median septum 
in the posterior quarter of the valve, and behind it two lateral septa, 
that fork from near the umbo ; the place of the lateral muscles is 
faintly marked. 
Sculpture . — Of fine concentric ridges, as with other forms of this 
species. 
Size. — Length of ventral, 2 mm. ; width, mm.; height, 1 mm to 2 
mm. Dorsal length. If mm. ; width, 2 mm. ; height, about J mm. 
Horizon and locality . — Fine, greenish grey calciferous sandstone 
of E. 2 c. at Dugald Brook, Escasonie. Not rare. 
