TROPIDOCARIS. PPP. 17 



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 curved, 

 convex to the lower margin. This latter ridge is much more 

 elevated and stronger, than the upper one, and is situated 

 midway between it and the lower margin of tlie 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 

 strife, 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 striae, which meet along the dorsal side. 



Telsox short and strong, produced into a spine of which 

 the entire length is not known. Lateral si)ines 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 lengtli of 15 

 mm., with a breadth of about mm. Two segments of 

 an abdomen measure respectively 3.5 and 6 mm. in length. 



This species is distinguislied from T. interrupia and T. 

 alternata by having only two prominent ridges on eacli 

 valve. The apices of the valves in T. interrupia iwa 

 mucronate, and in 7'. alternata every alternate ridge is 

 stronger and crenuliited along its summit, none of wliicli 

 characters appear in the j)re8ent species. 



The description of the abdomen is taken from ;i specimen 

 2 PPP. 



