28 



Cambridge, Mass., 1876, two specimens. 

 Madison, Wis., Sept., 1877, Third Lake. Eare. 

 Southampton, Mass., 1878. Bare. 



G-ENUS 5. 



Alona. Sars, 1862. 



This genus was first established by Baird, 1850, but was lim- 

 ited to a small portion of its former extent by Sars. I am not 

 sure that Alonella should have been separated from Alona, but 

 on the whole, prefer to keep the genus as Sars left it. 



SPECIES 1. 

 Plate II. Fig. 16. 



Alona ^ngulata. sp. no v. 



Length, 0.4 mm. Height, 0.25 m. 



Shell marked by rectangular meshes. 



The dorsal margin is considerably arched, terminating in a 

 more or less obvious angle at the hinder corner. The hinder edge 

 is convex, as is also the front margin. The ventral margin is 

 provided with plumose setas. The rostrum is pointed, as seen 

 from the side, and extends down nearly to the ventral edge of the 

 shell. The fornices are broad, the distance between their edges 

 being nearly equal to the greatest distance between the valves. 

 They are extended forward to the end of the rostrum. The shell 

 is obviously striated, the striae running obliquely downward and. 

 backward. Close inspection discloses a set of cross markings, 

 making the shell reticulated with oblong meshes. The post-abdo- 

 men is broad, and truncated at the end. It has a row of about 

 twelve teeth on each side, inserted a little way from its lower, dor- 

 sal, edge, and their points project behind it. Along the middle 

 of the post-abdomen, on each side, runs a row of very small 

 scales furnished with clusters of short hairs. The antennules are 

 rounded at the end, and the sense-hairs are set around the end, not 

 upon it. Thev have besides a flagellum. The basal joint of the an- 

 tennas reaches about to the edge of the fornix, the branches nearly 



