388 
BULLETIN OF THE NATURAL HISTORY SOCIETY. 
species with certain Hyolithidie has been made in an article contri- 
buted to the Royal Society of Canada (Trans. New. Ser. Vol. VII,, 
Sec. IV. p. 93). 
A study of layers of the shales of the horizon E. 3 e., studded with 
the valves of this species, failed to reveal any ventral valves, showing 
clearly a thickened callus. For the relationship of this species we 
have therefore to depend on the forms prima and crassa, both of 
which possess a narrow thickened callus. These show that these three 
forms are of the same genus as A. signata, but of a different species, 
and reveal a series in the upper Etcheminian Fauna parallel to the 
Signati of the lower fauna ; they are distinguished from the latter by 
their naiTOw visceral callus. The absence of a thickened callus in the 
typical form of A. proavia would seem to show that the pedicle in this 
form was slender and weak, and from the fact that this shell, above 
all its fellows, shows a perfect orientation in one direction, as im- 
bedded in the shale, there is a presumption that the pedicle was also 
long, enabling the animal to swing in the currents of the sea in which 
it lived. 
Often the ventral has an even slope along the back, but many old 
valves, especially long ones, show from two to three heavy concentric 
ridges, marking stages of growth of the shells. 
Interior . — The ventral valve of this species has a quite small tubercle 
in front of the foramen. Two-fifths from the apex of the ventral 
valve there is a shallow depression on the interior surface, which, by 
analogy with mut. prima should mark the position of the central 
muscle scars. On each side of the shallow depression a shallow groove 
runs forward toward the front of the valve. Some examples show a 
median and two lateral septa in front of the shallow depression. 
Faint ridges, running forward on each side of the front slope of the 
ventral valve, may indicate the position of vascular trunks. 
Interior . — The dorsal valve has inside, a median and two lateral 
ridges. On some valves the median ridge extends only so far as to 
divide the pits of the cardinal muscle ; in others it extends to the 
middle of the valve. A pair of median pits are sometimes visible 
near the end of the median septum, one on each side of it. 
