170 Proceedings of the Royal Irish Acadern 



n. The flagellum with less than ten jomts. 



(c). Abdomen broad 



(cT). Abdomen narrow. 



(<?). Left mandible with 2, right with 

 01117 1 pencil behind the cut- 

 ting parts (Sars) . 



{dr). Left mandible with 3, right with 

 2 pencils behind the cutting 

 parts (Sars) 



B. The outer ramus of the uropods broader than the 

 inner. 



L Tail-appendages projecting. 



(a). Able to roU into a ball .... 

 (6). Xot able to roU iuto a balL 



(J^). Flagellum with 1 joint only 

 (J*). Flagellum with 2 joints. 



* abdomen broad 

 ** abdomen narrow 



(S*). Flagellimi with 3 joints. 



* abdomen broad 

 ** abdomen narrow 



II. Tail-appendages not projecting. 



(c). Compound eye . . . ; . 

 {d). Simple eye . . . 



u. 



Hajplophthalmus. 



Trkhoniscus. 



Tricho7iiscoides. 



Cylisticv^s. 



PlatyartJiTus. 



Porcdlio. 

 Metoponorfhus. 



Outsells. 

 Philoscia. 



A rmadiUidium. 

 Elwma. 



We have included in this table the genus ligidium for convenience of 

 reference, though it has not yet occurred in Ireland. 



concrete example : let us suppose, therefore, that a specimen of Fhiloseia muMcrum is being studied. 

 On comparing the specimen with the characters giren, it will be seen at once that the outer ramus 

 of the uropod is broader than the inner ; it must therefore he referred to Section B. 



In this section we find that it belongs to the subsection {V), as it cannot roU up into a ball ; and 

 finding that it possesses 3 joints in the flagellum, it is erident that it must be referred to (V^), when 

 its narrow abdomen shows that it belongs to the genus Philosda. 



On turning, then, to the table (given at the head of each genus having more than one British 

 species in the following detailed sections) which difierentiates the various species of that genus, and 

 finding that the apex of the telson is sharp-pointed, there is no difficulty in identifying it as Fhilcscia 

 muicorum. It must be borne in mind that these tables are only intended as guides in distinguishing 

 the Britannic species at present known. 



