21 



ited sense, by the possession of teeth on the anterior margin. 

 From this fact, I have named the species P. denticulatus. 



SPECIES 5. 

 Plate I. Fig. 22. 



Pleukoxtjs Unidens. sp. nov. 

 Length, 0.85 mm. Height, 0.46 mm. 



Shell little arched on dorsal marerin. Lower oosterior corner 

 of valves rounded. A tooth just in front of the corner. Yalves 

 marked by striae. 



In its proportions, this species approaches P. straminius, the 

 back being comparatively little arched, so that the height is about 

 one-half the length. In the shape of the front part of the ani- 

 mal, there is also a close resemblance to P. straminius, and in the 

 relative length of the post-abdomen. There are, however, great 

 differences. The upper posterior angle is prolonged into a 

 projection, quite characteristic, seen, I believe, in no other species. 

 The lower corner is rounded, not angulated. Some distance be- 

 fore it is placed a single minute tooth. From this peculiarity, the 

 species has received its name. The bristles of the lower edge are 

 much larger in front. They become very small behind, and 

 seem to be smooth there instead of plumose. The valves are 

 marked by stria? One set occupies the upper half of the valves 

 and runs approximately parallel to the back. A second set runs 

 nearly parallel to the lower edge. The upper stria of this set is com- 

 plete, and tho-e of the upper set run into it where their curvature 

 will not permit them to reach the posterior margin without meeting 

 it. At the front part of the valve is a set parallel to the forward 

 edge. These meet the second set in an area which is irregularly 

 reticulated. The striation is very plainly marked. The post-ab- 

 domen is long and stout. The hinder end is truncated, but the 

 corner is slightly rounded off. There are two rows of 18 or 20 

 pointed, rather long, caudal teeth. The terminal claws have the 

 usual two basal spines, and are serrate. There are two small pro- 

 jections (one of which is shown in the figure) on the abdomen," 

 2 



