824: MR. H. G. JACKSON ON THE 



(2) Generic and Specific Characters. 



I have not found any marked sexual dimorphism to occur in 

 Ligidium, the difl'erence between the sexes being confined to the 

 modified pleopods of the male. In some species of Ligia the 

 sutures separating coxal plates from tergite are differently 

 marked in the sexes. It is interesting to note that no such 

 difference is to be found in this genus, and that the sutures are 

 marked, if at all, in an entirely different manner by fine semi- 

 circular grooves on the last four thoracic somites of both sexes 

 (text-fig. 7, a). 



The proportions of the body and the size vaiy remarkably 

 little. 



The colour of ijreserved specimens is of little value, but the 

 distribution of the pigment is sometimes characteristic and 

 always worth noting. 



The general sm-face of the body is remarkably smooth and 

 polished in all but two species, which have a roughened apjjcar- 

 ance owing to the presence of scales or knobs. 



Gephalon. The line of the epistome is continuous. Above and 

 between the insertion of the antennae it forms a downwardly- 

 directed V, which differs in length and sharpness in different 

 species. When prolonged and sharp it commonly projects 

 forward to form a slight triangular rostrum. This condition is 

 termed " produced " in the following descriptions (text-fig. 7, b). 

 The eyes are moderately largo in all species of the subgenus 

 Ligidium, and occupy the lateral corners of the head. In front 

 of the hind margin of the cephalon is a more or less deep trench 

 ("transverse groove "), opening to the cheek more or less behind 

 the eye on each side. The two pear-shaped pits on the forehead 

 of Ligia are represented in this genus by grooves (" frontal 

 grooves") which originate behind tlie eyes in the ti'ansverse- 

 groove, run forwards on the inner side of the eyes, and turn 

 inwards on the top of the head to run towards each other parallel 

 to the transverse groove. They never meet in the mid-line, but 

 end abruptly. The demarcation of these groo^^es has some 

 systematic value. (Text-figs. 3, 6 ; 7,5.) 



The thoracic somites vary little in general foi'm. The hind 

 margins of the first three are more or less straight, the fourtli is 

 slightly concave, and the remainder more deeply so owing to the 

 backward production of the lateral corners. These are nevei', 

 however, much produced. The first somite usuaJl}^ differs fiom 

 the remainder, as has been pointed out by Verhoeff (1901). In 

 many species the tergite is dinted on the postero-lateral corner 

 of each side, and the dint may take the form of a wide shallow 

 pit or of a shallow groove forming a " re-entr.ant " from the hind 

 border of the tergite (textrfig. 3, c & d). In either case it super- 

 ficially appears as if the tergite had been carelessly nipped by a 

 fine pair of forceps. This structure is referied to hereafter as 

 the " lateral depression.'"' When the depression is extended to 



