100 
Rl'l,LK'riN OF THE NATURAL HLS'l'OKT SOCIETY. 
Tho <lon«il valve of tins species (from E.3e.) has inside, a median 
and two lateral ridges ; on some valves the median ridge extends only 
so far as to divide the cardinal muscles; in others it extends to the 
middle of the valve. A strong pair of median pits are sometimes vis- 
ible near the end of the median septum. 
This is the oldest undoubted Acrothele detected in the Eopaheozoic 
rocks of Eastern Canada. Almost all show only the interior surface, 
or intermediate layers of the shell. One ventral has a corroded out- 
side, with traces of concentric ridges. 
Ackothele PROLES. 11. sp, PI. xvi, figs. to e. 
General form lenticular with the umbo of the ventral valve 
projecting. 
The length and breadth of the valves of this species are sometimes 
equal, though usually the width is somewhat greater. 
The ventral valve is convex on all the slopes, except close to the 
umbo, where it is slightly concave in front. The umbo is low, and is 
about one-seventh of the length of the valve from the hinge line, the 
area is about 1 mm. high, and the length of the hinge line nearly one- 
third of the width of the valve. Interior . — This has an obscurely 
lozenge shaped callus in front of the foramen, upon which at the pos- 
terior end is a small, more elevated portion. On each side of the 
callus is a pair of vascular ridges, marks of the advance of the central 
muscles. A pair of short ridges, near the hinge line, are of the nature 
of teeth outlining sockets for the articulation of the two valves. Faint 
curving ridges in the anterior part of the valve appear to be vascular 
trunks ; these fork toward front, and show eight or nine ridges with 
corresponding depressions along the anterior margin. 
The dorsal valve is more regularly lenticular, but more abruptly 
bent down behind than elsewhere, the umbo is depressed, and not 
easily recognized. Flattened valves exhibit costie radiating from the 
umbo, but not reaching the margin. Interior . — This shows a strong, 
broad median septum extending nearly half of the length of the valve ; 
at the front it fades away into fan-like ridges that rapidly sink to the 
level of the valve. On each side in the cavity of the valve, and 
extending nearly as far forward as the median septum and, diverging 
from it, is a pair of sharp vascular ridges. Ontside of these, on the 
rounded edge of the valve at the ends of the cardinal line, are a pair 
