69 
regularly but not greatly convex, depressed towards the cardinal angles. Area narrow, linear, 
without foramen; beak small. Dorsal valve concave. 
Surface shining, smooth, without radiating strie, but with very faint concentric lines 
of growth. 
‘The muscular impressions in the ventral valve extend more than half the length of the 
shell. The divaricator muscular imprints are deeply depressed, elongate-ovate, narrowing 
above, and somewhat widely separated below; while the occlusor impressions are narrow 
ovate spots on the thickened portion of the shell, and below these extends a narrow elevated 
band separating the other impressions. In the dorsal valve, the anterior and posterior occlu- 
sor impressions are small and not strongly defined. The cardinal process is slender, bifurcat- 
ing, with a narrow rounded ridge extending from its base down the middle of the valve. The 
interior of the shell, excepting the muscular impressions, is studded with prominent scattered 
granules or papilla. The crenulations on the hinge-line are rather distant but conspicuous.” 
(Hall, Pal. N. Y., Vol. IV., p. 104). 
Strophodonta nacrea is readily recognised by its general form and smooth 
surface, and, when the shell is preserved, by its pearly lustre. It is nearly 
allied to the Strophomena (Orthis) lepis of Bronn, from the Devonian Rocks of 
Europe ; but according to Hall, it is distinguished by being much less dis- 
Fig. 21. : ; : ; 
Strophomena _tinetly striated, and by having only about half as many crenulations on the 
nacrea, (Hall), of j jj 
$hé@ natural size. hinge line. 
Corniferous Lime- 
stone. 5 ; , j ; 
Locality and formation.—Rare in the Corniferous Limestone of Port Colborne. 
° 
82. STROPHOMENA RHOMBOIDALIS (Wahlenberg). 
This well-known species is so familiar to all students of Paleozoic Geology, and its char- 
acters are.so readily recognised, that it seems unnecessary to oceupy space here with a des- 
cription of its peculiarities, or a recapitulation of its ponderous synonymy. It may, as a 
rule, be determined at once by its rhomboidal form, the deep concentric wrinkling of the vis- 
ceral disc, and the abrupt geniculation of both valves towards the dorsal side at a point situ- 
ated at from one half to two-thirds of the length measured from the beak. S. rhomboidalis is 
known to have commenced its existence as early as the Lower Silurian period ; it is »n 
abundant Upper Silurian fossil; it is found throughout the Devonian ; and it even extends 
into the Carboniferous rocks. 
Locality and formation.—Common throughout the Corniferous J.imestone of Western 
Ontario. Also in the Hamilton group of the same district, though not known in the Hamil- 
ton series of the State of New York. 
Genus STREPTORHYNCHUS (King). 
The genus Streptorhynchus includes strophomenoid shells of a semi-circular or semi- 
elliptical shape, ‘ concavo-convex or plano-convex, and sometimes with both valves convex ; 
they are externally striated with rounded bifurcating thread-like striae, which are crossed 
by fine concentric lines; and in some forms the stronger striz are distant, with finer 
radiating and concentric striz cancellating the intermediate spaces. The ventral beak is 
sometimes produced and bent or twisted, and the fissure beneath the beak is closed or par- 
tially closed by a solid deltidium, while the area is subject. to great variation. A narrow 
area often exists on the dorsal valve, but this is not a constant character.” (Hall, Pad. 
N.Y., vol. ‘iv, p. 64). 
The genus Streptorhynchus is very closely allied to Strophomena proper, the most obvious 
external character by which the former is distinguished being the irregular twisting of the 
beak of the ventral valve. The genus is represented in all the great palaozoie formations, 
from the Lower Silurian upwards, and appears to be very abundantly represented in the 
Devonian Rocks of North America, so far as individuals are concerned, Prof. Hall, how- 
ever, refers all the Devonian forms of this genus to one exceedingly variable species, the 
Streptorhynchus Chemungensis of Conrad. Only one form of the group has been recorded as 
yet from the Devonian Rocks of Canada, namely, the S. Pandora of Mr. Billings. 
