ISOPOD GENUS LIGIA. 685 



2. The perteopods often possess well-marked processes or spines 

 in the male only, and also frequently bear a plate-like expansion 

 covered with file-like serrations on two or more segments of the 

 first two or three legs, as has already been remarked by Chilton 

 in Ligia oceanica (1899). The feature is constant enough to 

 justify the conjecture that it is of value in holding the female 

 in pairing. 



3. The sutures between coxal plates and terga are frequently 

 Avell marked on the dorsal surface of three or moi-e segments of 

 the thorax of the female, but in the male there is rarely more 

 than the faintest indication of the suture. This character will 

 be referred to in detail in the description of each species and in 

 the discussion on OeoUgia. 



4. The male is usually laa-ger than the female, but frecjuently 

 relatively narrower ; doubtless this difference is associated with 

 the brood-pouch of the latter. 



It follows from this that descriptions of species founded on any 

 of the above characters, in which the sex is not named, must be 

 looked upon with suspicion. As far as possible I have always 

 noted in the following pages characters in which the sexes differ, 

 or have stated on which sex the description has been based. 



(4) Generic and Specific Characters. 



Nearly every external character possessed by Ligia has been 

 used from time to time for systematic Avork. As these characters 

 are" of very unequal value, I will pass them in review and 

 endeavour to justify the characters on which I have based my 

 descriptions. 



The proportions of the body and the size vary with age and sex, 

 but bearing that in mind, they may be a valuable guide. 



The shape of the somites of the body differs very little in the 

 different species. As a rule the hinder margins of the first 

 three thoracic somites are transverse and the remainder pro- 

 gressively more and more curved, and the coxal plates are drawn 

 out into progressively longer backwardly- directed spines. The 

 first three or four somites of the thorax are about equal in length 

 and the others successively shorter. Unless there is any strongly 

 marked difierence, I have therefore omitted these points in my 

 descriptions. 



The general shape of the body cannot be described in other 

 terms than "elongate-oval" or the like, so it is unnecessary to 

 repeat it for each species. 



The colour in the case of preserved specimens, especially if they 

 have been kept in spirit a long time, is almost valueless. 



The surface of the body is always covered with minute 

 granulations, and the free margins of the somites are beset with 

 minute setae, but in some species the surface is notably smoother 

 than in others, 



