URECHINUS. 121 



were supposed to be separated from each other have become increasingly vague. 

 Mortensen's admirable study of the genus (1907. Ingolf Ech., pt. 2, p. 39-53) 

 threw a great deal of light on the group, but it is still a matter of no little uncer- 

 tainty how many really valid species there are. The form of the test shows ex- 

 traordinary diversity even in a single species and practically no reliance can be 

 placed on it for systematic purposes. Color is also variable and the tubercula- 

 tum of the test, the number of spines and the pedicellariae are not to be trusted. 

 Even the position of the periproct is not a reliable character. In the key given 

 herewith the character of the globiferous pedicellariae, which seem to be very 

 constant, and the number of genital pores, a character whose value is still unde- 

 termined, are exclusively used. In this way seven of the nine species hitherto 

 described can be distinguished, but I am obliged to consider the proposed species 

 Cystechinus purpureas A. Agassiz and Clark identical with loveni and Ur echinus 

 fragilis Koehler identical with wyvillii. It is a little odd that Mr. Agassiz and 

 I compared our specimens with wyvillii and not "with loveni, while Koehler does 

 not compare his species with wyvillii. 



The pedicellariae in this genus are very diversified, but the most character- 

 istic form, the globiferous, are unfortunately often very few and far between. 

 The ophicephalous are often exceedingly abundant but are not at all distinctive 

 of either genus or species. Mortensen's figures of an ophicephalous pedicellaria 

 (1907. Ingolf Ech., pt. 2, pi. ix, figs. 7, 10) supposed to be that of Cystechinus 

 clypeatus seem conclusive proof that the fragments on which such were found 

 were not the remains of an urechinid. Mortensen himself suggests this but 

 in my opinion there can be no question about it, unless the pedicellaria was 

 accidentally associated with the fragments. The tridentate pedicellariae in 

 Urechinus are numerous and diversified and frequently quite distinctive; 

 certain forms designated by Mortensen as "coarse tridentate" seem better 

 called rostrate even though their intergradation with ordinal tridentate can 

 sometimes be traced. The triphyllous pedicellariae are minute and rather 

 infrequent. The sphaeridia are notable for being quite rough or even prickly. 



Key to the Species of Urechinus. 



Globiferous pedicellariae with only a single terminal point. 



Genital pores 4; blade of globiferous pedicellariae curved inward but not bent . clypeatus. 



Genital pores 3; blade of globiferous pedicellariae distinctly bent inwards . . drygahkii. 

 Globiferous pedicellariae with 2 or more teeth around the terminal opening. 



Globiferous pedicellariae with the terminal opening having the lower margin with 



4-6 long slender teeth, the upper with none reticulatus. 



