38 THE VOYAGE OF H.M.S. CHALLENGER. 



The suture dividing the outer lamella of the opercida is accurately at right angles to 

 the long axis of the body in the female ; it is more oblique, slanting downwards in all the 

 male specimens that I have seen ; I am inclined, however, to think that this is rather an 

 individual variation than a mark of sex, inasmuch as nothing of the kind exists in any 

 other species. 



Serolis convexa has been figured by Cunningham ^ and by Studer,^ and as both these 

 figures, especially the latter, show plainly the characters of the species, I hardly think it 

 worth while to introduce another into the present Eeport. 



Serolis convexa can be easily distinguished from Serolis gaudichaudii by a number 

 of small characters ; the shape of the body is more oval in Serolis gaudichaudii ; in Serolis 

 convexa it is more pear-shajaed, owing to the greater length of the caudal shield, which 

 terminates in a more pointed extremity ; in both species there is a central and two lateral 

 carinse ; the latter are curved, and follow closely the lateral margin of the caudal shield 

 but at some distance from it ; in Serolis gaudichaudii these carinse, especially the two 

 lateral ones, are very faint ; in Serolis convexa the lateral carinse are strongly marked, and 

 terminate more or less abruptly in a short spiniform projection at the end of the middle 

 third of the caudal shield ; the median carina is only found in the anterior part of 

 the caudal shield, posteriorly it becomes obsolete ; another diSerence between the two 

 species is in the colour, and since this difference is quite constant in all the specimens 

 examined by me (five of Serolis convexa, four of Serolis gaudichaudii) it is worth noting ; 

 Serolis gaudicliaudii is distinguished by its dark brown colour, darker in the central 

 part of the body, and dotted all over with black spots of various sizes ; in Serolis convexa 

 the colour is of a uniform pale brown, hardly darker in the middle of the body than upon 

 the epimera. This description of course relates only to specimens preserved in alcohol. 

 Another species closely allied to Serolis convexa is Dana's Serolis plana, and from a 

 careful comparison of the figures and descriptions given of these two species I find it 

 almost impossible satisfactorily to separate them. The general shape of the body, the form 

 of the epimera, &c., is almost identical in the two. The eyes, however, ot Serolis plana are 

 stated by Dana to be conical in shape, whereas those of Serolis convexa, as in all other 

 species, are distinctly reniform. In Serolis plana " the articulation of opercular plates is 

 more nearly transverse than in Serolis gaudichaudii;" there is also a lateral tooth on the 

 caudal shield just below termination of carina ; the median carina is obsolete posteriorly, 

 and the fourth segment has a low jjrominence just inside of the epimeral suture. The 

 first character is, as I have already shown, of no use in distinguishing the species ; the last 

 character, namely, the presence of a low prominence to the inside of the epimeral suture 

 of the fourth segment, is well marked in the Challenger specimen of Serolis convexa ; in 

 this specimen all the thoracic segments have a slight prominence, almost indistinguishable 

 in the anterior and posterior ones, but conspicuous in the fourth segment, where it slightly 



1 Loc. cit., pi. ILx. fig. 3. 2 Isopoden gesamiuelt, &c., he. cit., Taf. i. figs, la, 16. 



