ridges extending nearly their entire length. The ridge nearer the dorsal line commences near the apex of the valve, following adjacent and parallel to the dorsal line to the base of the Cephalic area, and then diverging, extends almost to the posterior end of the valve, at about one-third the width of the valve from the hinge. The lower ridge also starts from the apex of the valve and is slightly carved, convex to the lower margin. This latter ridge is much more elevated and stronger, than the upper one, a11d is situated midway between it and the lower margin of the valve. The optic node is at the posterior extremity of a small Carina, between the two prominent ridges, extending to the base of the cephalic area.
Surface of the carapace marked by fine sharp irregular striæ. which are more conspicuous over the cephalic area and margins of the valves than on the thoracic portion.
Abdomen cylindrical, gradually tapering to the posterior end. But two segments are known, of which the anterior has a length equal to its transverse diameter; and in the posterior segment the length is twice the diameter. Lower edges thickened, anterior margins furnished with a rim for articulation with the adjacent segment. Surface marked by curved strife, which meet along the dorsal side.
Telson short and strong, produced into a spine of which the entire length is not known. Lateral spines not observed.
Test comparatively strong on the margins of the valves. the ridges of the carapace, and the ends of the abdominal segments.
The largest valves observed have a length of 39 mm., and a width of 14 mm. A small valve has a length of 15 mm., with a breadth of about 6 mm. Two segments of an abdomen measure respectively 3.5 and 6 mm. in length.
This species is distingushed from T. interrupta and T. alternata, by having only two prominent ridges on each valve. The apices of the valves in T. interrupta mucronate, and in T. alternata ridge is stronger and and crenulated along its summit, none of which characters appear in the present species.
The description of the abdomen is taken front a specimen 2 PPP.
