96 Wificonsrn Academy of Sciences, Arts, and Letters. 



is somewhat rounded. The terminal claws have very small basal 

 spines and no serration. 



The name is given on account of the deeply cut reticulations. 



Cambridge, Mass. Grlacialis, 1876. Eather scarce. Southamp- 

 ton, Mass., 1878. Not uncommon. 



SPECIES 4. 

 Plate I. Fig. 31. 



Pleueoxus Denticulatus. sp. nov. 

 Length, 0.5-0.6 mm. Height, 0.35-0.45 mm. 



Anterior margin of valves armed with small, backwardly pro- 

 jecting teeth. Rostrum not bent forward. 



The dorsal margin is very convex, descending rapidly to the 

 posterior margin, which is consequently relatively short. At the 

 junction of the posterior and ventral margins, there are two, three 

 or (usually) four teeth, or in young specimens none. Of these, the 

 upper tooth curves upward, the others outward, or the lowest a 

 little downward. There is a series of line teeth on the lower part 

 of the anterior margin, directed downward or backward. These 

 lie inside the row of sette. The shell is marked as in P. procur- 

 vus. There are also strios on the head, of which the lower run 

 parallel to the edge of the fornix, the upper parallel to the outline 

 of head. The rostrum is long, pointed, and curves backward. The 

 post-abdomen resembles almost exactly that of P. procurvus. 

 There is often a black pigment deposited io its lower part. The 

 male has a shorter rostrum, hairs instead of teeth on the post-ab- 

 domen, whose lower angle is rounded. Color, greenish or yellow- 

 ish. 



Glacialis, Cambridge, 1876. In muddy or clear water. Madi- 

 ison, 1877. Common. 



This species is allied to P. trigonellus (0. P. Mliller), from 

 which it differs in shell markings, and very greatly in the male. 

 (Yid Kurz. PL III, fig. 2.) P. Bairdii, Schodler (= P. trigonel- 

 lus, Baird) has the striae all parallel and extending over the shell, 

 a rounded and gibbous post-abdomen, and other differences. It 

 differs from these and all other species of Pleuroxus, in its lim- 



