THE DEVONIAN FOSSILS OF CANADA WEST. 265 
wards the sides, and with a broad shallow mesial depression towards 
the front. The dorsal valve is in general strongly convex; the hinge 
extremities rounded. The umbo and beak of the ventral valve are 
small, the latter sometimes a little elevated, but in general closely in- 
eurved. 
Large specimens, twice the size of those above figured, are not un- 
common in the Corniferous limestone. These are more elongate oval, 
or sometimes, owing to the wide straight hinge-line and projecting car- 
dinal extremities, the form is sub-triangular. 
The surface is covered with small radiating ribs, usually two or 
three in the width of one line. These are crossed by undulating con- 
centric lines of growth, which give to the ribs a nodose or rugged aspect- 
In large specimens from the Devonian rocks of the Hudson Bay 
Company’s Territory, the strize are much finer, there being four or 
five in one line. In others they are much stronger. The shell when 
partially exfoliated, exhibits a whitish silken or pearly lustre. Indi- 
viduals are sometimes found with the surface around the front margin 
covered with imbricating concentric lamelle. Length usually about 
one inch or a little less, sometimes three inches. 
Locality and Formation.—This species ranges from the base of 
Middle Silurian to the Devonian, and is found in most countries 
where these rocks have been recognized. In Canada West it occurs 
in numerous localities in the Clinton, Niagara, Oriskany, Corniferous,. 
and Hamilton formations. 
Genus StRICKLANDIA.— (Billings. ) 
SrRICKLANDIA.—(Billings.) Canadian Naturalist and Geologist, 
Vol. 4, p. 1382, April, 1859. 
RensseLz#r1a.—(Hall.) part. Twelfth annual Report of the Regents 
of the State of New York, p. 39, October 1859. 
Generie Characters.—Shell, usually large, elongate-oval, transversely 
oval, or circular, sometimes compressed ; valves nearly equal ; a short 
mesial septum in the interior of the ventral valve, supporting a small 
triangular chamber beneath the beak as in Pentamerus ; in the dorsal 
valve no longitudinal septa spires or loop yet observed; the whole 
of the internal solid organs, (so far as is yet known) consisting of two. 
very short or rudimentary socket plates, which support prolonged 
ealcified processes for the support of the cirrated arms. In all the 
