101 ,>'K hi ;..", 



as far apart at the front as half of the width of the valve. Another 

 pair of such grooves, about half as far apart as these, are faintly impressed 

 on each lateral slope of the valve. The visceral cavity is faintly marked 

 out by striated lines in the posterior half of the valve, and has an irregu- 

 lar arched front, projecting near median septum towards the front of the 

 valve. Faint vascular striae are visible on the median area towards the 

 front of the valve. Some examples from the horizon E 3 6 have a shorter 

 septum, and show the position of the central and lateral muscles closer 

 to the hinge line. The shells are more oblate. 



Sculpture. This is only known near the side of the valve, where it 

 consists of fine, closely set, more or less tuberculated ridges, parallel to 

 the margin. 



Size. Length and width equal, 7 mm. Depth of the ventral valve 

 about of a millimetre ; that of the dorsal 1 mm. 



Horizon and locality. All the horizons from E. 3a to E. 3e, (except E. 

 3d,) at Dugald Brook, Escasonie, N.S. 



The Acrotheles of E. 3c are much corroded, and do not show the 

 characters well ; they are mostly moulds from which the shell has been 

 exfoliated. One ventral shows well the hollow behind the hinge area, 

 and the foramen. 



Examples from the assises E. 3e. have in the ventral valve quite a small 

 tubercle in front of the foramen ; the visceral callus extends half of the 

 length of the shell, and at each side in front are sometimes seen pits of 

 the adductor muscles : on each side of the callus a groove run out toward 

 the front margin. Some examples show a median and two lateral septa 

 in front of the callus. Often the shell has an even surface to the margin, This the 

 but frequently there are a few strong concentric ridges that mark stages Acrothel 

 of growth. 



The dorsal valve of this 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 visible near the 

 end of the median septum. 



This is the oldest undoubted Acrothele detected in the Eopalseozoic 

 rocks of Eastern Canada. Almost all show only the interior surface, or 

 intermediate layers of the shell. One ventral has a "cor roded outside, 

 with traces of concentric ridges. 



