400 Proceedings of the Royal Physical Society. 



as U. elegans, E. norveyicus, E. melo, and E. Flemingii!' ^ It 

 will be obvious that a short differential diagnosis can only 

 be regarded as of provisional value, and specimens will often 

 have to be reserved for leisurely discrimination. Still it is 

 believed that the characters here given will prove satisfac- 

 tory in the majority of cases, so long as only British forms 

 are taken into consideration. 



With regard to distribution, I have given the British locali- 

 ties pretty fully, with the names of the authors who are 

 responsible for them. The exact references may be obtain-ed 

 from the bibliography. The British area has been accepted 

 as defined by Dr Norman^ in his recent paper on that sub- 

 ject. It thus includes most of the deep water investigated by 

 H.M.S. " Porcupine " and other vessels, as well as the Channel 

 Islands, though I am quite disposed to agree with those who 

 maintain that from a zoo-geographical standpoint they belong 

 rather to France than to England. The extra-British range 

 of each form is indicated generally, and in giving this I have 

 relied mainly upon the following authors : — Coast of the 

 United States, Verrill ; Greenland and Denmark, Liitken ; 

 JSTorway and Sweden, Sars and Diiben and Koren ; France, 

 Cailliaud and Fischer ; Mediterranean, Ludwig and Carus ; in 

 addition to the comprehensive " Ee vision " and " Challenger " 

 Eeport of Agassiz. 



The present catalogue contains twenty-nine species as 

 against twelve mentioned by Forbes, but, for reasons which 

 will be given immediately, the evidence for the occurrence 

 of some of these in British waters is so slight, that they 

 must be omitted from consideration in all comparative dis- 

 cussions. 



The list of species arranged systematically, according to 

 the classification of Prof. Martin Duncan, is as follows : — 



Order!. CIDAROIDA. 

 Fam. 1. CiDARiD^. 



1 . Cidaris (Dorocidaris) pajnllata. 



2. „ {Porocidaris) purpurata. 



^ Agassiz, "Blake" Echini, Mcin. Mus. Comp. Zool., x., p. 39. 

 ^ Ann. and Mag. Nat. Hist., (6) v., p. 345, 1890. 



