350 THE DEVONIAN FOSSILS OF CANADA WEST. 
Deseription.—This species resembles in shape some of the forms of 
S. inequistriata, but it can always be distinguished therefrom by the 
surface, which is covered with fine crowded, rounded or sub-angular 
strize of an uniform size, from eight to ten in the width of one-fifth 
of an inch, presenting that even aspect peculiar to the genus chonetes, 
and rarely exhibited by species of Strophomena. 
The ventral valve is usually extremely convex, most prominent in 
the upper half; the umbo large—obtusely rounded, overhanging the 
hinge line ; the cardinal angles compressed, reflected, forming short 
projecting scars ; on the cardinal edge from five to eight small spines, 
rarely preserved, their bases only being visible. Area of ventral 
valve, in old specimens, owing to the extreme incurvation of the car- 
dinal portion of the shell, inverted or brought under the body of the 
shell at right angles to the plane of the margin; in young specimens 
not so much inverted; its width about half a line, or a little more ; 
obliquely striated, the strize most distinct at the hinge line. Area of 
dorsal valve, very narrow—almost linear, the inner edge with a row 
of small pits for the reception of the serrated teeth of the opposite 
valve. 
The width of this species is usually about one-inch on the hinge- 
line but it sometimes attains the size of one-inch and a half. Length 
equal to, or one-third less than the length. 
The dorsal valve is seldom found, although the ventral valve is 
somewhat common. Of the former I have seen only two fragments, 
consisting of the hinge-line and a portion of the shell. One of these 
was in its natural connection with the ventral valve, and being silici- 
fied came away on immersion in acid; the divaricator processes are 
united at the base and separated above by a narrow fissure; they are 
grooved on the outside, the grooves converging towards the hinge- 
line so that when viewed from the side of the area they have the 
appearance of four small radiating ridges. 
The muscular impressions and foramen have not been observed by 
me. The triangular opening in the area represented by Fig. 121, 
may be the foramen, but it seems to me to be a fracture. 
Prof. Hall describes two species differing from each other in the 
size of the strie; in C. arcuata, “sixteen occupying the space of 
one-fifth of an inch, while only one-half that number can be counted 
in the same space on C. hemtspherica.”’ (LOth Regents, Rep., p. 117). 
Our specimens agree with the latter. 
