398 
HULLETIX OK THE NATUKAL HISTORY SOCIETY. 
only are they broader than the Acrotheles of the lower assises, but 
they are larger, some valves attaining 9^^ mm. in width. An exterior 
of a ventral which is nearly one-half wider than long, and an interior 
of a dorsal about a third wider than its length, are figured. 
In the examples from this horizon the ventrals show surface mark- 
ings, hinge area and foramen ; their interiors show crescentic grooves 
of the vascular trunks, and at the margin, prints of its branches. Some 
of the dorsals show the surface sculpture; others, which have the 
interior exposed, show median and lateral ridges, vascular lines, etc. 
A. AViA-PUTEis. n. mut. PI. xv, figs. 5 a and h. 
This seems to be a variety of ^1. avia. It differs in the possession 
of a pair of pits, one of which lies on each side of the space between 
the foramen and the visceral callus, partly overlapping each. The 
visceral callus is quite short in this form and has but little prominence. 
The ridges on the surface of the valve are more regularly concentric 
than in the type, and more sharply cut ; about ten are found in the 
space of one millemetre. The cardinal area is curved forward towards 
the top, and finely striated. The foramen is about a fifth of the length 
of the valve from the cardinal line, and the front of the callus about a 
third. Vascular trunks and branches are visible on the surface of the 
ventral valve as in the type. The dorsal valve does not sensibly differ 
from that of A. avia. 
Size . — The largest valve seen was 8 mm. long, and about the same 
width. 
Horizon and localily. — Found in the Bretonensis shale (E.3cf.) at 
Gregwa Brook, Escasonie, Cape Breton. Fre<juent. 
Acrothele abavia. n. sp. PI. xv, figs. 3a to d, and 4a and h. 
Outlines of the valves nearly circular. Length of the hinge line 
less than a third of the diameter of the valve. 
Ventral valve rather flat, with the umbo slightly raised. The umbo 
is about one-quarter of the length of the valve from the cardinal line.^ 
Interior. — In the examples known tlie horizon E.3a., the interior is 
smoothly moulded, except along the front slope, wliere faint vascular 
grooves may l)e detected, but in those from E.3A. a visceral callus is 
faintly outlined, with a swelling on the middle ; some valves here have 
