G'84 MR. H. G. JACKSON OKT THE 



to endeavour to carry on, from the place at which it was left by 

 Budde-Lund, the task of making a critical revision of the'Isopoda 

 Teri-estria,' with the aid of his collection and other material in 

 the Museum. 



I should like here to pay tribute to the generosity with which 

 Dr. Caiman has always put his unrivalled knowledge of the 

 Crustacea at my disposal during this investigation ; my grateful 

 thanks are also due to my wife for many of the figures illus- 

 trating this paper and a great deal of other tedious work, and to 

 Mr, Frank Forty for his excellent figures of L. jngnientata and 

 L. perkinsi. 



(2) Introduction. 



Budde-Lund recognised seventeen species in the genus Ligia, 

 five of which he had not seen and therefore did not describe. 

 Since the publication of ' Isopoda Terrestria ' several new species 

 have been created, three of which still stand, some of the doubtful 

 species have been set on their feet, and the genus Geoligia has 

 been created to contain two purely terrestrial Ligiidse. Budde- 

 Lund's descriptions were always in Latin and, valuable as they 

 are, were often extremely sketchy ; and no serious attempt was 

 made to describe thoroughly any species of Ligia until Sars 

 in 1899 gave his classic diagnosis and figures of Ligia oceanica. 

 Later, in 1901 and 1916, Chilton did an equally admirable 

 service for Ligia nova3-zeala7idice and exotica. Not all modern 

 descriptions have improved greatly on their predecessors, although 

 Miss Richardson has given careful accounts of the American 

 species of Ligia. 



Having had the privilege of examining an extended series of 

 forms, I have been in a very favourable position for weighing the 

 relative importance of various characters in discriminating 

 between species. In descriptions of new species, characters are 

 again and again cited as specific which are found on examination 

 to be generic, but not i^eferred to — either being considered as un- 

 important or not having been noticed — by previous authors. For 

 example, such a careful and reliable writer as Chilton gives 

 " transverse depression just posterior to the angle of the eye, 

 interrupted in the middle " as a specific character of Ligia novce- 

 zealandice, when, in fact, there is no Ligia of any species I have 

 seen in which this " depression " is absent. 



(3) Sexual Characters. 



The very marked sexual differences have been a stumbling- 

 block in the recognition of genuine species. 



The secondary sexual characters in which the sexes differ are 

 as follows : — 



1. The antennse of the male are usually relatively longer than 

 those of the female and are frequently stouter, though not in all 

 species. 



