MARINE ISOPODA OF NEW ENGLAND, ETC. 415 



The first pair of legs are robust, and in the males may be large and 

 much elongated ; they are in both sexes of our species i^owerful organs 

 of prehension, being strongly chelate. Like the remaining i^airs of legs, 

 they have only five movable segments, uuless an articulated spine at 

 the extremity of the fifth segment is to be regarded as the true dactylus. 

 On the other hand, the basal segment in many specimens presents indi- 

 cations of a short segment at its distal end, as if really consisting of the 

 united basis and ischium. If this latter supposition be the true one, the 

 hand of the first pair of legs is formed, as might be expected, of the pro- 

 podus and the dactylus ; the propodus is thickened and provided with 

 a digital process stronger than the curbed dactylus, which closes against 

 it ; the digital process bears toward the tij) a few stout, bristly setae. 

 These legs are attached to the under side of the united head and first 

 thoracic segment below the branchial cavity, and are directed forward. 

 They are capable of but little lateral motion, and are nearly in contact 

 below, especially toward their bases, which cover and partly conceal 

 the organs of the mouth and the bases of the antennae. The second pair 

 of legs are very slender in comparison with the first, and are more slender 

 than those that follow. Their basal segments are flattened, somewhat 

 elongated, and usually bent with the convexity outward, in adaptation to 

 the basal segments of the first i)air of legs, which they partly embrace. 

 The last three pairs of legs have their basal segments swollen. 



The pleon consists, in our species, of five or six segments, and bears 

 three or five pairs of strongly ciliated pleopods of the ordinary form, 

 and fitted for swimming, and also a pair of uropods, consisting of a large 

 basal segment bearing one or two rami. This ramus, or the inner one 

 when there are two, is articulated and composed, in our species, of from 

 two to six segments. The outer ramus may also consist of more than 

 one segment. Like the antennulae and antennae, the uropods are pro- 

 vided with setae, which are often elongate. 



In the young the seventh pair of legs are not developed, and, accord- 

 ing to MiiUer, the pleopods are likewise wanting and the uropods have 

 less than the adult number of segments. 



This family has been the subject of special research by Fritz MiiUer, 

 Spence Bate, Dohru, and others, to whose writings reference may be 

 had for further description of their anatomy and development. Their 

 proper place among the Crustacea cannot be regarded as settled, though 

 the opinion of Fritz jVliiUer that they represent an ancestral type of 

 Isopoda is probably the best offered as yet. According to Dohrn, they 

 present in their development affinities with Asellus, lAgia, and Guma. 

 Gegenbaur associates his Tanaida with the Podophthalma rather than the 

 Udriophthalma. 



Our species of this family are sharply divided into two genera, for 

 which I have, after some hesitation, adoped the names Tanais Aud. and 

 Edw. and Leptochelia Dana. I have not been able to see Audouin and 

 Edwards' E6sum6 d'Entomologie, in which the genus Tanais is said to 



