ADDITIONAL NOTES ON THE CAMBRIAN OF CAPE BRETON 387 
Size. — Length of the ventral valve in the largest examples, 3 mm. ; 
width, 2 mm. The dorsal valves in both diameters is 2 mm. Depth 
of the ventral valve from the beak mm. ; that of the dorsal at the 
middle, h mm. A great majority of the valves are smaller and of the 
size given in the original description. 
Horizon and locality . — In the Assise e, common (and less common 
in d where it is larger) of the upper Etcheminian, Dugald Brookr 
Escasonie, Cape Breton. Very thin shells are found in Assise f. 
There is a good deal of variation in the form of the ventral valves 
of this species. The majority are of the dimensions given, but some- 
times the width of the valve is equal to the longest diameter. Also 
the concentric furrows of growth are in some examples so profound as 
to give the ventral valve, when slightly distorted obliquely, the appear- 
ance of a minute Raphistoma. 
This species differs from all others of the genus known to me, ex- 
cept L. inflata of the Protolenus fauna, in the overhanging apex, which 
in the typical form projects one-quarter beyond the base of the valve, 
but in the variety from Assise /, one-third beyond. As a result of 
their form, the ventral valves of this species, in place of standing erect 
like many of those of the genus Acrotreta, rest on the dorsal side, on 
the layers of the shale in which they are imbedded, and except for 
their strong convexity might be mistaken for those of a minute 
Lingulella. As they occur scattered over the layers of the shale they 
also strongly recall the ordinary aspect of the conical teeth of fishes, 
brilliant with black enamel. 
No described species of Acrotreta is as small as the more abun- 
dant valves of this species, though A. gemmula of the Protolenus 
fauna approaches it in that respect.* 
Sir William Dawson has called attention to the resemblance in 
structure between the shells of Hyolithidfe and the Brachiopoda, and 
has compared the ventral valve of a Brachiopod to the tube of a 
Hyolithes. Had Sir William been acquainted with this species he 
would have found it a good example for comparison. This will be 
seen if the ventral valve be so oriented, as to make the areal side cor- 
respond to the ventral side of a tube of Hyolithes. The dorsal valve 
with its round form and excentric umbo, with radiating lines, also re- 
sembles the operculum of a Hyolithes. A detailed comparison of this 
* Trans, Roy. Soc. Can, Vol. xi, p 87, pi. xvi, figs 2 ato d. 
