A.DDITIOITA.L NOTES ON THE (^AMBRIAN OF CAPE BRETON .383' 
The dorsal valve is more regular in form than the ventral, but also 
often fjuite tumid. The umbo is low and close to the margin. Interior 
— This possesses a shallow median septum extending to the middle of 
the valve ; on each side of the septum, at the hinge line, are pits for 
the cardinal muscle. A pair of diverging grooves in the posterior half 
of the valve mark the position of the lateral muscles. 
Both valves have thickened borders and are flattened along the 
lateral margins. 
Considering the variableness of this form one might be disposed to 
think it a mutation of A. signata, and it is so classed here ; but the 
following differences are apparent : The visceral callus of the ventral 
valve is broader and not so distinctly impressed, and the groove at 
the posterior end project farther backward. In A. signata the callus 
does not have the strong bounding ridges that this frequently is seen 
to have. The cardinal area in this form never has the extreme over- 
hang that marks A. signata^ and the back part of the ventral valve is 
not produced. 
ScMlpture. — The surface is smooth, but a strong lens reveals fine 
concentric ridges at intervals on the surface of the shell. 
Size. — Ventral, length, 2^ mm. ; width, 2| mm. ; depth, TJ mm. 
Dorsal, length and width, 2\ mm. ; depth, f mm. 
Horizon and locality . — Fine grey shales in the volcanic beds of 
the Coldbrook Group at Dugald Brook, Escasonie (C. B.), N. S. 
AcROTHYRA (signata) SERA. P1‘. XIII, figs. 3 a-f. 
Valves corneous, thick, especially the ventral. General form 
orbicular, with the umbo of the ventral projecting. 
Ventral valve nearly circular in outline, and with the back either 
straight, or slightly hollowed near the apex, and rounded down toward 
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 someNvhat 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 towards 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 
