10 Messrs. E. W. L. II(.lt and W. M. Tattersall on 



Geous MysideteSj \hAt aiiil Tattersall. 



Ml/si (fetes, of which we give a full diagnosis in a paper 

 now in the press*, difEers chiefly from Mysideis, G. O. Sars, 

 iu having the pleopods rudimcntarv in hoth sexes. The 

 telsou is cleft, the cleft armed with spines, and the inner 

 uropod has a row of spines from the otocyst almost to the 

 extremity. The antenna! scale is setose on both margins. 

 The first aud third thoracic limbs have endopods of the usual 

 type and serve respectively to distingiush the genus from 

 Heteromt/sis aud Mijsidella, which resemble it in the other 

 characters mentioned above. 



Mysidetes posthon, sp. n. 



Form robust. Carapace produced in front, with a short 

 and very obtuse rostrum ; emarginate behind, leaving the 

 last thoracic segment exposed. Pleon with the firs^t five 

 segments subequal in length, sixth segment barely once and 

 a half as long as the fifth. Eyes large, globose; pigment 

 brown. Antennular peduncle with the outer distal corner of 

 the basal joint produced into a long narrow process, which 

 extends beyond the distal extremity of the second joint. 

 Antenna! scale lanceolate in shape, setose all round, about 

 four times as long as broad. Antenna/ peduncle very little 

 more than half of the length of the scale, third joint shorter 

 than the second. Mouth-parts in all respects as in Mysideis. 

 First and second thoracic limbs with the endopods substanti- 

 ally as in Mysidopsis. Remaining thoracic limbs having the 

 tarsus of the endopod composed of six to eight joints ; the 

 endopods of the last pair much more slender than in the 

 preceding pairs. Male f/enital process very long and narrow. 

 Pleopods of both sexes rudimentary, consisting of a single 

 short ramus bearing at its base a short external lateral process 

 tipped with setaj. *Telso'fi about as long as or a little longer 

 than the sixth segment of the pleon and about twice and a 

 quarter as long as wide at base ; tapering gradually to the 

 apex, in which is a cleft equal in depth to nearly one fourth 

 and in gr. atest width to about one seventh of the total length 

 of the telson ; cleft armed with about eighteen teeth on each 

 side ; apex of the telson with a pair of spines on each side of 

 the cleft, the outer being the longer; lateral margins armed 

 throughout with about seventy spines, which become 

 arranged in series towards the apex. Inner uropods with 

 about twenty-six spines from the otocyst to the last fourth 



* ' Fislieries, Ireland, Sci. luvest.,' 1904, v. 



