270 THE DEVONIAN FOSSILS OF CANADA WEST. 
RuynconewiA Teruys.—N. Sp. 
Fig. 20. Fig. 21. Fig. 22. 
Figs. 20, 21.—Side views of the specimens of R. Tethys. Fig. 22.—Front view. 
Description—In this species the body of the shell, excluding the 
beaks, is transversely sub-oval ; from the beaks the sides diverge at an 
angle of about 110°, and are nearly straight, or gently concave, for 
about one-third the length of the shell, below which they are regularly 
rounded ; front margin broadly rounded, nearly straight in the middle 
for the width of the mesial fold. Ona side view the outline is ob- 
liquely sub-conical, the base obtusely rounded, the lower three-fourths 
of the ventral valve straight, forming an angle of about 100° with the 
ventral half of the base; the dorsal contour gently arched, and the 
dorsal half of the base rounded. The mesial sinus of the ventral valve 
commences at about mid-length, and increases only slightly in depth, 
until the front margin is reached, when the middle portion of the shell, 
for the width of the sinus, is rather suddenly bent towards the dorsal 
valve, with an abruptly rounded curve, and forms about three-fourths of 
the depth of the base. There are five angular ribs in the sinus, their 
extremities deeply forked to receive the projecting points of the grooves 
between the ribs of the dorsal valve; on each side of the sinus there 
are six principal ribs, their extremities a little turned upwards ; above 
these, three or four smaller ones. The upper part of this valve is 
narrowly convex, with a prominent umbo, and incurved beak; in the 
lower half, a little concave towards the lateral margins, owing to the 
elevation of the extremities of the ribs. Dorsal valve with all the 
central region depressed convex, the margin of -the shell on each side 
of the mesial fold in front abruptly bent towards the ventral valve ; the 
umbo obtuse, divided along the middle for a short distance by a narrow, 
barely perceptible depression, the beak strongly ineurved under that 
of the ventral vaive. The mesial fold dies out at two-thirds the length, 
the shell (of the fold) at the lower extremity bent towards the ventral 
valve at an obtuse rounded angle, and extending about one-fourth across 
the base. On the mesial fold there are six ribs, the middle four most 
