MARINE ISOPODA OF NEW ENGLAND, ETC. 337 



supporting two lamellae, and two or more of the anterior pairs are ciliated 

 with fine plumose hairs. The inner lamella of the second pair of pleo- 

 pods bears, in the adult males, a slender style articulated near the base 

 of the inner margin and varying in length and structure in the different 

 genera and species. The pleopods, besides their branchial office, are 

 also of importance in locomotion, being used for swimming, which is a 

 frequent mode of progression in this family, and is often performed with 

 the back downward. 



The females are usually broader than the males and carry their eggs 

 and young in a pouch, on the under surface of the thorax, formed of 

 four pairs of plates, attached to the coxal segments of the second, third, 

 fourth, and fifth pairs of legs, and overlapping along the median line. 



The known Isopoda of this family on the coast may be most easily 

 recognized by the presence, underneath the pleon, of a two-valved oper- 

 culum, opening like a pair of cupboard doors, and by the first three 

 pairs of legs being more or less prehensile. Our genera may be distin- 

 guished by means of the following table: 



i dissimilar, last four pairs not prehensile Chiridotea, p. 337 



an nnsB | ^^^ articulated, clavate Erichsonia, p.C54 



(. short and rudimentary Ei'elys, p. 357 



Chiridotea Harger. 

 Chiridotea Harger, Am. Jour. Sci., Ill, vol. xv, p. 374, 1878. 



First three pairs of legs terminated by prehensile hands, in each of 

 which the carpus is short and triangular, the j)ropodus is robust and the 

 dactylus is capable of complete flexion on the propodus; antennae with 

 an articulated flagellum ; head dilated laterally ; abdominal operculum 

 vaulted, with two apical plates. 



The two species of this genus found on our coast agree further in the 

 following particulars : The body is short, the length being only about 

 twice the breadth, and the outline of the head and thorax together is 

 subcircular. The anterior part of the lateral margin of the head is pro- 

 duced and deeply lobed, the eyes thus appearing dorsal instead of lateral ; 

 posteriorly the head is deeply received into the first thoracic segment. 

 The antennulse are proportionally large, equaling or surijassing the pe- 

 duncle of the antennae. The external lamella of the maxillipeds (see pi. 

 lY, figs. 18 and 21) is large and broad and the palpus consists of only 

 three segments, of which, however, the last two are each composed of 

 two coalesced segments, that are separate in the European Ch. entomon. 

 Of the two segments thus formed, the terminal is quadrate or rhomboid 

 in outline, with rounded angles and is smaller than the preceding, which 

 expands distally toward the articulation between the two. 



The thorax is deeply excavated, in front for the head and behind for the 

 abdomen, so that the thoracic segments are much longer at the sides than 

 along the back, when measured parallel Avith the axis of the animal. The 

 22 F 



