146 THE DEVONIAN FOSSILS OF CANADA WEST. 
Fig. 57. 
Fig. 56. 
Fig. 56.=Side view. Fig. 57.—Dorsal view of a specimen with a truncated fron margin. 
Deseription.—Transversely sub-oval; greatest width about the middle 
or a little above; the front margin sometimes extended into a short, 
broadly-rounded linguiform projection, and sometimes nearly straight, 
or even a little concave for about one-third the width. Both valves 
moderately convex; the ventral valve usually with a shallow mesial 
sinus, or depression, which becomes obsolete before reaching the beak ; 
dorsal valve with a broad slightly elevated mesial fold. Beak and 
umbo of ventral valve of moderate size, the former incurved, and 
perforated at the point by a circular aperture. The umbo of the 
dorsal valve is small and neatly rounded, the beak buried beneath that 
of the opposite valve. Surface marked by sharp concentric ridges, 
which are sometimes so greatly developed as to cover the whole shell 
with thin overlapping scale-like plates. 
Length from nine to fifteen lines; width a little greater than the 
length. 
This well known fossil has a very wide geographical distribution, 
being found in the Devonian rocks of Russia, Germany, France, Spain, 
England, and America. 
It varies a good deal in form, according to the sediment im which 
it is found. Where the shell is thin, the middle of the front margin 
is straight or concave, as in Fig. 57; but the thick-shelled indi- 
viduals have the front margin more or less pointed. Some think 
our species different from the European form ; but others, such as 
De Verneuil, Roemer, Lyell, Sharpe, and others, who have compared 
specimens from both sides of the Atlantic, have pronounced them to 
be identical. 
Locality and formation.—Occurs in the Corniferous Limestone in 
the Township of Cayuga, and in the Hamilton Shales at various 
places in the Township of Bosanquet. 
Collectors.—A. Murray, T. Richardson, J. De Cew. 
