6 ARBACIA DUFRESNIL 



Arbacia Dufresnii 



! Echinus Dufresnii I'.i.ainv.. 1825, D. X. Sc. Nat 0. 



! Arbacia Dufremii Gkay, 183j, Proc. Zool. Soc. London. 



Pl.I.f.,. ;. 



Additional specimens of this species, collected in the Straits of Magellan by 

 the Hassler Expedition, confirm the value of the specific characters employed 

 to separate A. Dufresnu from A. stellata, its nearest ally. It is quite remark- 

 able that in the lew specimens existing in the British Museum ami in our 

 collection, there should lie two specimens having five anal plates, instead of 

 the normal number of four in the other species of the genus. This may ex- 

 plain the rudimentary fifth anal plate of some specimens of Parasalenia -ecu 

 by Troschel.* He was. however, mistaken in considering Parasalenia. on this 

 account, the young of some Echinometra. The young of Parasalenia is what 

 I had called in some collections Cladosalenia, but which I have since found 

 to he only small Parasaleniae. In all the Kchinometrae known to me the anal 

 Bystem is covered by a large number of anal plates, long before the specimens 

 attain even one fifth the size of the specimen of Parasalenia figured in /'/. 

 Ill', f. /, .', of the Revision of the Echini. I cannot agree with Troschel in 

 separating Arbacia into two genera, for which he has proposed the names 

 Echinocidarifl and Pygomma, based upon the position of the ocular plates. 

 The ocular plates. ;is is well known among young Echini,! have at first no 

 connection whatever with the abactinal system, ami in the Echini proper 

 and Echinometradae, they either reach the anal system, or are excluded 

 from it in specimens of nearly the same size. The only family thus far 

 where the position of the ocular plates seem- of generic value is in the 

 Diadematidae ; hut there the ocular plate is connected with some of the anal 

 plates, in such a manner a- to Beparate all the genital plates, and the separa- 

 tion is not limited to one or two plates, as in the families above mentioned. 

 Professor Troschel, during my visit at Bonn, in 1870, called my attention to 

 his views of the value of this character, hut I have been unable, after a 

 careful examination of the large material at my command, to satisfv myself 

 that this feature has the importance he would assign to it; owing to the 



great variation in the position of the ocular plates in the species, he includes 



in his genus Pygomma, where it is often impossible to decide if the ocular 



plate really reaches the anal Bystem or simply spreads apart the genital 



plates. 



• Trobodi i. Die Familie dor Ecbinocidaridcn, 1878, Wicg. Archiv. 

 t A. Aoamiz, Embrjrologj "i Bchinodcnna, Hem. Am. Acad., 186ft 



