FRESH-WATER CRUSTACEA OF THE UNITED STATES. 657 



small, and tipped with four or five setiform spiiiules. Telson 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 antennulte are about 5'^'^ long : the flagellum being twice as long as 

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

 antennae 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 antenuulse are longer than the antennae; and the 

 flagellum of the antennulae is composed of sixteen to nineteen segments, 

 while that of the antenute 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; Edwards, Hist. Nat. 

 des Crust., vol. iii, p. 147, 1840 ; 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 antennulae cylindrical, much larger than the next two, which are, 

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

 ing the peduncle of the antennae. Antennae 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 antennae and antennulai 

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

 segments. 



Thoracic segments increasing in breadth po.steriorly ; all behind the 

 first segment with the anterior angle produced and gradually turning 

 more and more backward in the posterior segments. Epimera becoming 

 conspicuous on the posterior segments, which have their lateral borders 

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

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

 obtusel}^ I)ointed 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 



