( ^ ) 



From a revision of these various systems, it appears, that putting 

 out of sight such arbitrary characters as mere colour and number, divi- 

 sional characters of most importance are : the comparative development 

 of the external antennsB or rather of its filament ; the mode of inser- 

 tion and development of the last pair of abdominal feet, and perhaps 

 we may add the form of the maxilliped ; this last, however, is a cha- 

 racter so difficult of application that it were, perhaps, as well to leave it 

 out. Two very important sets of characters have been omitted or 

 overlooked, which must be taken into account in any classification 

 which seeks to form natural groups ; these are, — the characters drawn 

 from the form of the head; and from the epimeraUy as they are generally 

 called. These characters, being permanent in their structure, presenting 

 differences easily caught at, and running also in parallel arrangement 

 throughout the groups, I would propose as the basis of the classification, 

 which, till a better shall arise, I would suggest as that which appears 

 most natural and most in accordance with homological affinities. 



The characters, then, which must be considered in detail are : — 



1st. The amount of development of the so-called ''epimerals" ox coxa 

 of the posted or cephalo- thoracic and abdominal appendages. 



2nd. The form of the head as regards the absence or presence of what 

 are called the frontal or median, and lateral lobes. 



3rd. The relative structure and development of the antennae, and 

 especially of the terminal filament, or, as we will call it, ** the tige'^ of the 

 external or superior antennse. 



4th. The structure and position of the last pair of abdominal false 

 feet, their relations to each other and the last abdominal ring. 



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 ^^ EpimeraU.^^ 

 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 coxae of the ambulatory and swimming organs ; 

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



