Al>mTIONAL NOTES ON THE CAMBRIAN OE CAPE BkETON. 
397 
From the posterior part of the shell several faint radiating ridges 
extend toward the front margin. 
Sadpture. — The surface of the valves is marked by irregular, con- 
centric, rounded ridges, that frequently anastomose ; and the front of 
the ventral valve and the sides of the dorsal valve have a number 
of radiating ribs. The sculpture is very variable ; on the central 
part of the dorsal it shows an irregular network of low, rounded 
ridges ; on the sides of the valve these ridges are more regular in their 
course; arid on the margins, especially of old shells they are stronger 
and more continuous. There is also much variation in the distinctness 
of the features of the interior, both of the dorsal and ventral valves, 
the smoother shells being thinner. There is no trace on the interior 
of the dorsal valve of the ridges on its lateral slopes. 
Size. — Length 9 ram. ; width 10 ram. or more. Depth of the two 
valves together 2 mm. or more. Horizon and Locality. — In assises d 
and e of the Upper Etcheminian (E. 3 d and e) Dugald Brook, Esca- 
sonie, N. S. 
As the outer layer of this shell is thin and fragile, the strong 
inner layer is the one most commonly exposed, and might be thought 
the real surface. The outer surface has an ornate sculpturing, while 
that of the second layer is smoothed. 
This species of Acrothele is distinct from all others by the long 
tubercle or callus in front of the pedicle opening. The ribs of the 
outer surface of the lateral slopes of the dorsal valve are peculiar to 
it. A. Matthewi and its varieties have no such ribs. A. Matthewi- 
prima has a granulate-latticed surface, but no lateral or anterior ribs. 
A. Matthewi-costata also has a granulated surface, and ribs on the front 
of the ventral, but none on the sides of the dorsal valve. The varieties 
of A. Matthewi also have the foramen nearer the cardinal margin 
than is the case with this species. 
Examples of this species occurring in the Assise E. 'id differ from those 
of A. ahavia occurring with it, in the thinner corroded valves, larger 
size and oblate form ; they are doubtfully referred to this species, for 
the ventral valve is more concave in front of the umbo than are the 
typical shells occurring in Assise E. 'ie. It does not Hake at the middle 
layer of the shell as A. ahavia of the same assise docs. 
In Assise E. id valves appear, which, by their oblate form and surface 
markings, may, without much doubt, be referred to this species. Not 
