MARINE ISOPODA OF NEW ENGLAND, ETC. 313 



three genera, and a species of another genus (Asellus communis Say) 

 is common in the fresh-water ponds and streams of New England. 

 The genus Limnoria Leach has been regarded by modern writers as be- 

 longing to this family, but will be found in the present article in the 

 Limnoriidce (p. 79). There remain then to be considered the genera 

 Asellus Geoffroy,* Jcera Leach, Janira Leach, and Munna Kroyer, which, 

 as represented in our waters, may be further characterized as follows : 



The head is well developed, and in Munna is of large size ; the body is 

 usually depressed or but slightly arched, except that the pleon is vaulted 

 in Munna. The eyes are present in our species though not through- 

 out the family. The antennulae beyond the basal segment are slender 

 and are always much shorter than the antennae, which are elongated 

 and composed of a five-jointed peduncle and a slender multiarticulate 

 flagellum. The first three peduncular segments are short 5 the last two 

 elongated. The parts of the mouth are protected below by a pair of 

 maxillipeds with large external lamellae and five-jointed palpi. Within 

 the maxillipeds are two pairs of maxillae of the ordinary form ; the outer 

 or second pair delicate and three-lobed at the tip ; the inner lobe being 

 formed by the i)rojecting basal segment, while the two outer lobes are 

 articulated ; all three lobes are provided with curved spiniform setae. 

 The inner, or first, pair of maxillae present two narrow lobes ; the outer 

 lobe broader and more robust than the inner, and armed with robust 

 curved spines, while the inner is tipped with much weaker setae. The 

 mandibles (see fig. 12 &, pi. Ill) are provided with one or two acute den- 

 tigerous lamellae (d) at the tip, usually a comb of setae and a strong molar 

 process below (m), and a triarticulate palpus (p}. This latter organ is, 

 however, wanting in the genus Mancasellus Hargert from the Great 

 Lakes and other fresh- water localities of North America. 



The seven segments of the thorax are distinct from the head and from 

 each other, and differ but little in general appearance throughout. The 

 legs are mostly slender and elongated, except that the first pair may be 

 more robust and better fitted for prehension. In our marine species 

 the dactylus, at least behind the first pair of legs, is short and armed 

 with two small claws or ungues, while the propodus is capable of con- 

 siderable flexion on the carpus. 



The segments of the pleon are united into a single piece, which is scuti- 

 form above, flattened or but little arched, except in Munna, and bears, at 

 or near the tip, the biramous uropods, which are, however, nearly obsolete 

 in Munna. The pleou often shows more or less trace of its compound 

 character in imperfect transverse sutures on the dorsal surface near the 

 base, and below it is excavated for the pleopods, the posterior pairs of 

 which are delicate and branchial in their nature, while the anterior pairs 



* "Hist, des lus. t. ii" (Edw.). For information in regard to the common European 

 form of this genus the reader should consult the admirable work of G.O. Sars, Hist, 

 nat. des Crust, d'eau douce de Norvege. 



t Am. Jour. Sci., III. vol. xi, p. 304, 1876. See, also, op. cit., vol. vii, p. 601, 1874, and 

 This Report, part ii, p. 659, pi. i. fig. 3, 1874. 



