260 THE VOYAGE OP H.M.S. CHALLENGER. 



tubercle, posterior to which, between the coxse of the fourth and fifth pairs, lies the 

 thelycum (fig. 1'"), which consists of two divisions, the anterior being an elevated cordi- 

 form plate and the posterior a circular prominence surrounding a central depression. 



In the male the fifth pair of pereiopoda carries- the foramen of the vas deferens on 

 a prominence on the inner side of the base of the coxa, and the first pair of pleopoda sup- 

 ports a petasma (fig. 1") that folds like a double-fluked anchor, the arms of which extend 

 laterally and lie flat against the ventral surface of the pereion and its appendages. The 

 second pair of pleopoda is two-branched ; the inner branch supports at the base a 

 button-like process. The succeeding pairs are only generic in character and become 

 smaller in succession posteriorly. The posterior pair, which contributes to form the 

 rhipidura, has no tooth projecting on the outer margin, but the external ridge terminates 

 at the external distal extremity ; a second ridge commencing near the centre of the 

 basal joint traverses the plate diagonally, and meets the external margin at the same place 

 as the obsolete diaeresis. The inner plate is nearly as long as the outer, and is longi- 

 tudinally traversed by two central ridges ; one, commencing at the articulation, continues 

 in a straight line to the apex ; the other commences at the inner margin near the base, 

 whence in a curved line it turns inwards and then continues parallel with the central 

 ridge to the apex. The telson runs to a sharp point, the sides are depressed and 

 fringed with hairs, amongst which is one small spine on each side, visible only on 

 close observation through a hand-lens, whence it somewhat suddenly narrows. The 

 dorsal surface is flattened, and traversed in the median line by a longitudinal groove. 



Observations. — This species bears a close resemblance to Penseus velutinus, Dana. 

 The two forms, moreover, are generally found associated in the same localities, and on 

 casual observation might be mistaken for one another. Both are tomentose, carinated 

 on the posterior somites of the pleon, have a number of teeth on the rostrum, and the 

 ophthalmopoda resemble each other. 



But in Penseus velutinus the flagella of the first pair of antennas are very short, not 

 being longer than the last two joints of the peduncle, while in Penseus anchoralis they 

 are as long as the peduncle itself. There is no fissure between the supraorbital and the 

 antennal tooth in Penseus velutinus, like that in Penseus anchoralis. In Penseus 

 velutinus there is no fissure on the margin of the first somite of the pleon, but in Penseus 

 anchoralis there is one that bisects it subequally. In Penseus velutinus the carina on 

 the third somite of the pleon is double, or longitudinally grooved ; in Penseus anchoralis 

 it is entire. In Penseus velutinus the fourth and fifth somites are cleft at the posterior 

 extremity, and the carina elevated in front of the cleft to a small tooth ; in Penseus 

 anchoralis the cleft is scarcely appreciable, and there is no tooth. The scaphocerite in 

 Penseus velutinus is furrowed on the lower surface, and the diagonal ridges are not 

 conspicuous, while in Penseus anchoralis the lower surface is polished, and the ridges are 

 larger and conspicuous. In Penseus velutinus the first pair of pereiopoda has a long 



