FRESH- WATER CRUSTACEA OF THE UNITED STATES. 657 



.small, and tippprl with four or five setiforin spiniiles. Telsou two-thirds 

 as broad as long, tapering- very slightly toward the entire and slightly 

 arcuate posterior margin, which is armed with about ten slender spin- 

 ules. 



In the largest male seen, 13'"™.5 in length, (excluding the antennae,) 

 the.auteuuul?e are about S"^"" long : the flagellum being twice as long as 

 the peduncle, and composed of about twenty-two segments, while the 

 antenna? are stout, fully 6""" long, and the flagellum as long as the 

 peduncle, and composed of fifteen segments. All the females and most 

 of the males which I have seen are much smaller, being 6"^™ to 8""" in 

 length, and in these the antennulse are longer than the anteunpe; and the 

 flagellum of the antennul;Te is composed of sixteen to nineteen segments, 

 while that of the antenniie has only eight to ten. 



The only specimens which I have seen were found in wells at Middle - 

 town. Conn., and were sent to me by Mr. G. Brown Goode. 



ISOPODA. 



Family Asellid^. 

 ASELLUS COMMUNIS Say. (Plate I, fig. 4.) 



Journal Academy Nat. Sci. Philadelphia, vol. i, p. 427, 1818; Edwai'ds, Hist. Nat. 

 des Crust., vol. iii, p. 147, 1340 ; DeKay, Nat. Hist. New York, Crust., p. 49, 1844, 

 A. vulgaris? Gould, Invertebrata of Massachusetts, p. 337, 1841. 



Head with the anterior margin nearly straight ; external angles 

 obliquely truncated; sides nearly parallel, with a small, prominent lobe at 

 the posterior angle; hinder margin somewhat rounded and shorter than 

 the anterior margin of the first thoracic segment. Eyes near the middle 

 of the lateral margin, oval, convex, with many facets. Basal segment 

 of the antennuliTB cylindrical, much larger than the next two, which are, 

 however, well marked as peduncular segments ; flagellum nearly equal- 

 ing the peduncle of the antennie. Auteuuie with three short basal seg- 

 ments, which are together about equal in length to the fourth ; last 

 peduncular segment equal in length to the third and fourth together ,• 

 flagellum much longer than the peduncle, extending, when bent back- 

 ward, about to the base of the abdomen. Both antenmTe and antennulaj 

 with scattered hairs, which are larger and stouter on the peduncular 

 segments. 



Thoracic segments increasing in breadth posteriorly; all behind the 

 first segment with the anterior angle produced and gradually turning 

 m )re and more backward in the posterior segments. Epimera becoming 

 conspicuous on the posterior segments, which have their lateral borders 

 emarginate and the posterior angles rounded. Pleon (abdomen) sub- 

 orbicular, slightly excavated at the insertion of the caudal stylets and 

 obtusely pointed between them, ciliate along the entire margin, as are 

 the head and the lateral borders of the thoracic segments. 



Mandibles with conspicuous triarticulate palpi, of which the first seg- 

 S. Mis. 71 42 



