FKESH-WATER CRUSTACEA OF THE UNITED STATES. 651 



appendages on the sternuiD of the third, fourth, and fifth segments of 

 the thorax, as in P. Hoyi, bat apparently none upon the second. 

 • Leugth from the front of the head to the tip of the telson, G ""'\ 



Of this species, I have seen but a single specimen, which was dredged 

 with the last species in Lake Michigan, in 40 to GO fathoms, off Eacine^ 

 by Dr. Stimpson, from whom it was received under the manuscript name 

 of Gammarus Jilicornis. 



This species differs remarkably from all the heretofore-known species 

 of Pontoporeinw, in the excessive elongation of the flagella of the an- 

 tennuliB and antenuic, a character which might be regarded by some 

 naturalists as of generic value. The very close agreement with P. affiiiis 

 and Hoyi in all other parts of the animal, however, seems to indicate a 

 very close affinity with those species, especially the latter ; and as this 

 one peculiarity is very likel}^ only a sexual character of the old males 

 of the species, I retain the species in the genus. The mouth-appendages 

 seem to agree perfectly with those of the species just mentioned. The 

 singalar armature of the lateral margins of the third segment of the 

 abdomen is not peculiar to this species, but is almost exactly repeated 

 in P. affinis, P. Hoyi, and the marine species, already mentioned, from 

 the Gulf of Saint Lawrence, and is probably common to the genus, 

 although it seems to have been overlooked till now. 



Family GAMMARiDiE. 



Gammarus limn^us Smith. (Plate II, fig. G, lateral view; fig. 7, 

 dorsal view.) 



Gammarus lacustris Smith, American Journal of Science, 3d series, vol. ii, p, 453, 

 1871 ; and Preliminary Report on Dredging in Lake Superior, p. 1023, 1871. 



Eyes small, slightly elongated. Antennul^e about as long as the 

 thorax ; first segment of the i)eduncle about as long as the second and 

 third together ; flagellum about twice as long as the peduncle, composed 

 of about twenty-five elongated segments, furnished with few and minute 

 setfe, or hairs ; secondary flagellum short, scarcely, if at all, longer than 

 the terminal segment of the i^eduncle, composed of two to four seg- 

 ments, of which the terminal one is very short. Antennfe as long as, 

 or a little shorter than, the antennulte; ultimate and penultimate seg- 

 ments of the peduncle nearly equal in length, naked above, and fur- 

 nished with a few short hairs, or setae, arranged in three or four small 

 fascicles ; flagellum considerably shorter, to nearly as long as the 

 peduncle, composed of about twelve segments, furnished with a few 

 short hairs. 



Legs of the first pair in the male with the carpus short, triangular ; the 

 propodus a little less than twice as long as broad, much narrowed distally ; 

 the palmary margin slightly concave in outline, continuous with the 

 posterior margin, with a narrow lamellar edge, and furnished with a 

 few long hairs and with two ong, obtuse spines near the middle, and 



