276 THE DEVONIAN FOSSILS OF CANADA WEST. 
opens out-into the visceral cavity. The lower side of this space is 
nearly straight, and the two lower angles rounded. The lateral margins 
of the space are usually sub-parallel in the lower half, while in the 
upper half they approach each other, and meet above to form the 
upper angle. In some specimens thé space is more nearly triangular, 
and it would appear, therefore, that its form is a little variable. The 
length of the space is about one-third the whole length of the ventral 
valve, and its width at the lower margin a little less than its length. 
The lower margin is situated a little above a line drawn across the 
shell at mid-length. The lower three-fourths of the space is striated 
longitudinally, and divided into two equal portions by an obscure 
median groove. 
On each side, at the base of the foramen, there is a short, stout 
tooth. The dental plates below the teeth extend but a short distance 
into the visceral cavity, when, becoming suddenly much diminished in 
height, they form a low ridge along the upper margin of the muscular 
space. The upper part of the muscular space is deeply excavated into 
the substance of the shell, which is very thick and solid in the rostral 
half. 
I have not seen the interior of the dorsal valve. 
Externally this species resembles A. fwmida (Dalman,) but the 
muscular impressions in the interior of the ventral valve are widely 
different in the two species. 
Locality and formation.—Rama’s farm, near Port Colborne, and at 
many places in the County of Haldimand. Corniferous Limestone. 
Specimens, with the valves united, are rare, but the upper part of the 
ventral valve, with the umbo and beak preserved, is not uncommon. 
Collectors —A. Murray, J. De Cew, E. Billings. 
Atuyris Maia.—N. Sp. 
Description.—Smooth, ovate, or sub-rhomboidal. Ventral valve 
strongly convex, most gibbous in the upper half; umbo prominent, 
large, giving to the shell, on a side view, somewhat of the aspect of a 
Pentamerus ; beak strongly incurved, but not touching the surface of 
the dorsal valve ; a shallow, concave mesial sinus, extending from the 
front all the way to the beak. Dorsal valve moderately convex, with 
a convex mesial fold, which becomes obsolete near the beak. Length, 
from one inch to one inch and a half. The proportional width is 
variable. In: some specimens it is exactly equal to the length, but in 
