ETCHEMINIAN FAUNA OF CAPE BRETON. 20$ 



The dorsal valve has radiating ridges on the lateral slopes — a 

 character unique to the species. The interior of this valve has, at the 

 umbo, the usual " nail-like process," as Prof. Hartt called it, viz., the 

 median ridge, on each side of which are impressions of the umbonal 

 and lateral muscles. The group of central muscles are somewhat in 

 front of the middle of the valve. Faint impressions of vascular, and 

 of radiating ridges, are also visible. 



Sculpture. — The surface of the valves is marked by fine, irregular, 

 concentric rounded ridges, that frequently anastomose. These ridges 

 are more regular in their course on that part of the shell which is near 

 the margins. 



Size. — Length 9 mm., width 10 mm. or more. 



Acrotreta proavia, n. sp. PI. HI. figs. 2a to / 



Shell substance calcareo-corneous. Ventral valve obliquely coni- 

 cal, with a prolonged beak. Cardinal area narrow ; a tubercle below 

 the foramen, which opens behind the beak. The beak of this valve- 

 has a heavy overhang beyond the aperture (nearly half its width). 

 The dorsal valve is orbicular, convex, but somewhat flattened towards 

 the front, the interior has two strong muscular pits near the hinge 

 line, spreading vascular impressions, and a broad low median septum. 



Sculpture. — A strong lens reveals a series of concentric striee on 

 the surface of the shell, of which there are about twenty in the space 

 of a millimetre. A still stronger magnification brings to view a gran- 

 ular ornamentation ; occasional rows of coarser granules are seen to 

 be parallel to the concentric striae. 



Size. — Length of ventral valve 2 mm., width 1| mm. The dorsal 

 valve is 1^ mm. long. 



This species differs from all others in its overhanging apex. From 

 having this form the ventral valves are found lying prostrate on the 

 layers and when their dark shining inner layers are exposed look very 

 like the conical teeth of fossil fishes. 



They also are not unlike a long conical Lingulella, and may show 

 us a step by which the genus Acrotreta was differentiated from some 

 such form as Lingulella. 



