The most conspicuous difference is, that the oblique fold separating the thorax and peduncle is more plainly developed, projecting at the point corresponding to _h_ in fig. 1, Pl. V, 8/1000ths of an inch; in the middle the fold is notched; it can be traced more easily than in _I. Cumingii_, running beneath and parallel to the basal edge of the mouth, to the ventral margin of the body. In the mouth there is hardly any difference; the maxillæ, however, have two notches even plainer than in the hermaphrodite _I. quadrivalvis_, or than in the male _I. Cumingii_, but the depth of such notches is always a variable character; there are also more spines on the edge in the male of the present species, than in _I. Cumingii_. Both mandibles and maxillæ in the male _I. quadrivalvis_, are larger than in the male _I. Cumingii_, to a greater degree than the larger proportional size of the body in the former will account for; and this, likewise, is the case with these same organs in the hermaphrodite _I. quadrivalvis_ compared with the female _I. Cumingii_. The tubular olfactory orifices are situated in the same peculiar position as in the hermaphrodite, and as in both sexes of _I. Cumingii_: they are 1/500th of an inch in diameter, and about as thick as one of the lower segments in the rami of the sixth cirrus. The thorax, as in the male of _I. Cumingii_, is quite rudimentary, and serves as a mere flap to protect the mouth. In the three specimens carefully examined, the posterior cirri had each only one ramus, whilst the anterior cirri generally had two: in one specimen, one of the rami in the anterior cirrus was formed of five segments, and the other ramus of three segments, both rami being supported on a uni-articulated pedicel; but on the opposite side of the same individual, the anterior cirrus was represented by a mere knob. The longer ramus of the anterior cirrus, in the best-developed individual, barely exceeded in length the mandibles measured along the line of the teeth! In one specimen between the bases of the posterior cirri, there were two perfectly distinct caudal appendages; these, like the cirri, are in a quite rudimentary condition; one was 5/1000ths of an inch in length, and consisted of three segments, the upper edges of which had short spines; the other was shorter, uni-articulated, but spinose. In a second specimen, these appendages were quite aborted. Close under them, on the inside or towards the mouth, (that is, in the normal position,) there was a rudimentary but quite distinct penis, with the apex projecting freely, and with the sides distinguishable from the ventral surface of the thorax, for the length of 1/1000th of an inch: the corium lining this little penis made the terminal orifice plainly visible. The vesiculæ seminales lie in the usual position, and are conspicuous; they are slightly tortuous, with their ends blunt: in the specimen so well preserved in spirits, they were filled with a mass of spermatozoa,