73 



DESCRIPTION OF THE FOSSILS OF THE COLDBROOK 

 TERRANE. 



ACROTHYRA SIGNATA-PRIMA. Plate I, figs. 1 O-g, 



Test (calcareo-)corneous ; valves tumid. Ventral valve variable in Acrothyra 

 form, longer than broad, often quite tumid, with the posterior half prima. 

 straighter than the anterior, which in some examples is strongly arched 

 down toward the margin. Hinge area variable in height, beak sometimes 

 overhanging the hinge, sometimes withdrawn from the perpendicular. 

 Interior There is a visceral callus from one-quarter to one-third of the 

 length of the valve, wider in front than behind, bordered by vascular 

 grooves ; the central depression is greater toward the apex than toward 

 the front. Outside of the aforesaid grooves is another and a shorter pair, 

 more widely diverging ; traces of the lateral muscle scars are seen outside 

 of this latter pair of grooves. 



The dorsal valve is more regular in form than the ventral, but also 

 often quite 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 car- 

 dinal muscle. A pair of diverging grooves in the posterior half of the 

 valve mark the position of the lateral muscles. Both valves have thick- 

 ened borders and are flattened inside along the lateral margins. Consid- 

 ering 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 grooves at the posterior 

 end project farther backward. In A. siynata 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. 



Sculpture. The surface is smooth, but a strong lens reveals fine con- 

 centric ridges at intervals on the surface of the shell. 



Size. Ventral, length, 2J mm. ; width, 2J mm.; depth, 1 mm. Dor- 

 sal, length and width, 2| mm. ; depth, f mm. 



Horizon and locality, Fine gray shales in the volcanic beds of the 

 Coldbrook Group at Dugald Brook, Escasonie (C.B.), N. S. 



ACROTRETA PAPILLATA-PRIMA, n. mut. PI. Ill, figs. 1 O-C. 



Only the ventral valve of this form is known. This is wider than long, Acrothyra 

 tumid, with the cardinal area vertical. Interior. In this the visceral 



