53 
The three radiating, almost imperceptible, impressions are 
not features of Cyrtodonta , but as the muscle scar, the position 
of the beak, and the general outline of the shell agree with 
Cyrtodonta , it is placed provisionally in that genus. 
Horizon and Locality . Chazy: Aylmer limestone at 
MacLaren landing. 
Modiolopsis compacta n. sp. 
Plate IV, figure 3 
Average length 20 mm.; average width 12 mm.; average 
depth of convexity 4 mm. 
The size is rather variable. One specimen procured is only 
16 mm. in length and 10 mm. in width; another specimen is 14 
mm. in width, and, were the complete length preserved, must 
have been about 25 mm. Valves very inequilateral. The 
anterior portion of the shell is short and narrowly rounded ; the 
posterior is much broader and longer. The hinge line forms an 
angle with the curve of the posterior margin at about the middle 
of the shell. The valves are deepest at about one- third from the 
anterior margin. The beaks are small incurved, and placed at 
about a quarter of the distance from the anterior end. The 
umbonal region is prominent, almost sharp in casts, particularly 
in the anterior portion; specimens having the test preserved 
are more uniformly rounded. A shallow sinus is present, com- 
mencing at the beak and broadening out until at the ventral 
margin it only slightly alters the convexity of the marginal 
outline. The surface is marked with concentric growth lines. 
The muscle scars are faintly impressed. 
Of the Chazy Modiolopsis already described it seems most 
closely allied to Modiolopsis fabaeiformis Raymond but differs 
from it in size, in the proportionate length of the hinge line, and 
the length of the posterior portion, also in the position of the 
most convex portion of the shell. 
Horizon and Locality . Chazy: Aylmer limestone at 
MacLaren landing. 
