686 MR. H. G. JACKSON ON THE 



The cephalon has a transverse groove on its hind border and 

 two somewhat pear-shaped post-orbital pits ("interrupted de- 

 pression "of Chilton). There are three transverse grooves on the 

 epistome in all species. 



The eyes of the saine species have been described by different 

 authors as "moderate," "large" or "small." Compared with 

 some nearly related genera, the eyes of any Ligia would be 

 large : it is therefore unnecessaiy to do more in a specific descrip- 

 tion than compare one species with another. 



The abdomen (metasome) has the first two somites covei^ed by 

 the last thoracic and without the drawn-out pleural plates possessed 

 by the remaining somites. The difference in width between 

 abdomen and thorax is more marked in some species than in 

 others, but I cannot consider it a good character, as it is distinctly 

 affected by the preservation of the specimen ; when the body is 

 flaccid and the anterior abdominal somites are more exposed, the 

 difference is exaggerated. In many species there are indications 

 of lateral oblique carin^e on the third, fourth, and fifth abdominal 

 somites. I have noted the character when it is conspicuous. 



The terminal segment (telson -f- 6th abdominal somite) varies 

 slightly with every species and is, therefoie, a valuable cha- 

 racter. The posterior margin may be produced into spines or 

 blunt processes. It may bear a median process which niay be 

 acute or almost obsolete ; the corners may be drawn out into 

 postero-lateral ^yrocesses. On each side above the point of insertion 

 of the uropod the margin may bear two processes (sometimes 

 little more than undulations), the inner and othter accessory 

 processes (PI. II. fig. 10). 



The key given on p. 689 is based in the first place on the shape 

 of the posterior margin between the two inner accessory processes. 

 If a median process is present, the margins on either side of it, 

 between it and the inner accessory processes of each side, are 

 always, however slightly, concave. Long usage has sanctioned for 

 this type of telson the inaccurate term " triangulate." 



On the other hand, an " arcuate " margin is smoothly convex 

 between the inner accessory processes. 



It is conceivable that cases may arise which are intermediate 

 between the two types, but the material described below presents 

 no difficulties. 



The appendages are of great systematic value. 



T\\e first antennce are vestigial throughout the genus. 



The second antennce differ greatly in length, in the character of 

 the segments of the flagellum and in the relative length of the 

 peduncle and flagellum.. I have followed Miss Richardson in 

 describing the length of the appendage in relation to the somites 

 of the body, but I cannot follow her in attaching importance to 

 the relative length of the segments of the peduncle. The 

 number of segments of the flagellum is useful as a general indi- 

 cation, but as it sometimes differs on the two sides of the same 

 animal, it must not be taken too seriously. 



