BRITISH ASSOCIATION". 263 



Before entering on these, the following nomenclature of parts must 

 be stated : — Head, : all the parts anterior to and including the first ring, 

 or, in other words, the homologue of the carapace of the Brachyura. The 

 term cephalo-thoracic refers to the portion of the body included between 

 this point and the posterior border of the eighth ring, counting the head 

 the first : this homologizes, we believe, with the pereion of Spence 

 Bate ; while the remaining six rings are referred to as the abdomen, ho- 

 mologizing with the abdomen of the Brachyura, and the pleon and tel- 

 son of Spence Bate. The appendages attached to the last abdominal 

 ring are cited as the last pair of abdominal feet, for such is evidently 

 their nature ; the so-called epimerals stand as coxa, and the filament of 

 the antennae as tige. 



I. — Amount of Development of the "Epimerals." 



The " epimerals" first claim our attention. For the determination 

 of the true homological relations of these we are indebted to the re- 

 searches of C. Spence Bate, F.L. S., among the Amphipoda, who, in the 

 first Part of his Report on the British Edriophthalmia, published in the 

 Report of the British Association for 1855, has proved most conclusively 

 that these so-called epimerals are truly and homologically the first joint, 

 or, as he calls them, the coxce of the ambulatory and swimming organs ; 

 and although, in the case of the posterior pairs of appendages in the 

 Isopods under consideration, their true nature, when present, is often- 

 times obscured, yet, by a little care in the examination, it may be at 

 once seen, though somewhat disguised. 



Their presence in the cephalo-thoracic rings seems to be constant in 

 all the genera I have examined, or of which detailed accounts are ex- 

 tant ; though sometimes, as in Ligidium, the suture which divides them 

 from the edge of the true epimerals is so faintly marked as not to be ap- 

 preciable ; hence I suspect that Lereboullet laboured under an error in 

 supposing them absent in Ligidium Persooni, for an examination of his 

 own figures of the articulation of the ambulatory legs shows that they 

 must be present, from the position of the articulation of the first joint, 

 which arises from beneath a ledge which evidently is the coxae of the 

 limb : it is hazardous to speak positively on this point without an exa- 

 mination of specimens, but in Armadillium, Ligia, Philoscia, Philougria, 

 Porcellio, and Oniscus, I find them well marked, and they are figured 

 or recorded in Itea, Titanethes, Tylus, and several other genera. In 

 the abdominal segments there may be some question of their presence, 

 but what are usually called the posterior angles of the rings a very lit- 

 tle examination shows really to be these coxae. This is remarkably 

 Well seen in Ligia, and, judging from Dana's figures in Tylus above 

 all, where they appear to be regularly articulated to the rings in the 

 eight, i. e. the first abdominal segment. Dana appears to have over- 

 looked this character altogether ; there is some great confusion in his 

 figures : many of the species certainly are not belonging to the genera 

 to which he has referred them, — I may instance Oniscus nigrescens, 0. 

 maculatus, and 0. pubescens, which are most certainly not Oniscus, but 

 most probably Philoscia judging from the figures of the false feet. 



