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BULLETIN OF THE NATURAL HISTORY SOCIETY. 
at a wide angle, and limit the area occupied by the lateral muscle 
scars. The position of the central group of muscles in front of the 
callus is not clearly defined. 
The dorsal valve is orbicular and the umbo depressed. Two broad 
obscure ridges radiate from the umbo to the sides of the valve. When 
viewed sidewise the valve is seen to be bent down both at the anterior 
and posterior ends. Interior . — The most prominent feature is the 
median septum, which is usually visible from one-sixth of the length 
of the valve from the back, to the middle of the valve. A pair of 
diverging grooves originate at the hinge line, and forward, towards the 
sides of the valve, divide off the space occupied by the impression 
made by the lateral muscles. Midway between these grooves and 
the median septum, are two faint vascular ridges. The margins of 
both valves are thickened and flattened. 
Sculpture . — This consists of fine concentric ridges with smooth 
intervals between ; the known surface is smooth and shining, but 
there are fragments of what appears to be an outer layer, with a dull, 
minutely granulated surface. The surface is cften ridged with growth 
lines, especially toward the anterior margin. 
Size . — Length and width of the valves equal, mm. The depth 
of the ventral is mm.; that of the dorsal, 1 mm. 
Horizon and locality . — The Assise Ic of the Etcheminian at Dugald 
Brook, Escasonie (C. B.) N. S. 
This differs from the type in the shorter and wider shell, upright 
hinge area, wider visceral callus, and straighter back of the ventral 
valve. From A. signata prima in the more regularly conical form 
of the ventral valve. 
Acrothyra signata-tarda. pi. XIV, figs. 1 a-d. 
Only the ventral valve known. This is tumid, with a broad low 
umbo, and convex on the median line. Interior . — Distinguished by 
two short prominent grooves that end abruptly, short of the end of 
the callus ; the callus is narrow and has a low ridge along the middle; 
it ends 1 \ mm. from the hinge, and the two lateral grooves are about 
1 mm. apart. Outside of the two grooves above named are low 
■crescentic ridges in front of the lateral extensions of the hinge line, 
that enclose the scars of the lateral muscles. 
