MARINE ISOPODA OP NEW ENGLAND, ETC. 411 



and placed well forward at the sides of tbe head. The antennulse (pi XIT, 

 fig. 76 a) are shorter than the head and slender, sparingly hairy, with a 

 short, few-jointed flagellum. The autenuse (pi. XII, fig. TGh) are also 

 slender, with the first segment apparently composed of two united ; the 

 second segment short ; the third and fourth longer, nearly cylindrical and 

 followed by a slender few-jointed tiagellum. The jaws (pi. XII, fig. 76 c) 

 are elongate and turned upward at the apex, irregularly and bluntly 

 toothed near the base within, and somewhat carinate on the outer side 

 near the middle, the carina ending rather suddenly in a tooth-like pro- 

 cess of the jaw as seen from above. The under surface of the head is 

 deeply and broadly grooved longitudinally, and this groove is covered 

 by what appear to be the transformed first pair of thoracic legs (pi. XII, 

 fig. 76 d). They are in the form of a semi-oval plate on each side, attached 

 near the base of the external side and strongly convex and ciliated on 

 the innep side, where they overlap. This plate is truncated at the apex, 

 where it bears a small oval lamella ; on the surface of the large i)late 

 are three large, oval, semi-transparent areas. Within these plates is 

 another pair of organs, consisting of a large basal segment and an artic- 

 ulated series of four flattened ciliated segments. These may be regarded 

 as the maxillli)eds, with a four-jointed palpus. 



The first thoracic segment is indicated above only by a faint sutural 

 line near the posterior margin of the large head. It is followed by five 

 very distinct segments, of which the first two are perhaps most distinct, 

 short, and strongly tuberculated, especially along their posterior mar- 

 gins. The third free segment is broader than the second, square at the 

 sides, with two broad lateral elevations. The fourth free segment is 

 somewhat rounded in front, with its chitinous integument apparently not 

 calcified along the median line. The fifth free segment is narrower than 

 the preceding and produced at the sides around the small last thoracic 

 segment and the base of the pleon. The legs are nearly alike through- 

 out, somewhat hairy and spiny. 



The pleon is slightly dilated at tlie middle, with the angles of the 

 segments salient. The last segment is acutely triangular, ciliate behind, 

 surpassed by the uropods, which are also ciliated with a few bristles ; 

 both rami are slender, the inner a little broader than the outer. The 

 pleopods (pi. XII, fig. 78 e) cofisist of two slender elongate lamellse, the 

 inner longer than the outer, attached to a basal segment and not ciliated 

 in the adults of our species. 



Length 4.4'"'"; breadth 1.3"""; color dirty yellowish brown above, 

 lighter below. This form is Anceus americanus Stimi)son. 



The adult female (pi. XII, fig. 77) differs from the male principally in 

 the following characters : The body is smooth and tapers behind and 

 before, but is much swollen medially, where the segmentation becomes 

 obscure, and the thoracic region seems converted into a sack for the 

 reception of the eggs, plainly to be seen through the transparent integu- 

 ment. The head is comparatively small and subtriangular, emarginate 



