6 A.RBACIA DUFRESNII. 



Arbacia Dufresnii 



! Krhliuis Dufresnii Blainv., 1825, D. N. Sc. Nat. O. 



! Arbacia Dufresnii Gray, 1835, Proc. Zool. Soc. London. 



PL I./. 3, 4. 

 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. Dufresnii from A. stellata, its neai'est ally. It is quite remark- 

 able that in the few specimens existing in the British Museum and in our 

 collection, there should be two speciuiens having five anal plates, instead of 

 the normal number of four in the other sjiecies of the genus. This mav ex- 

 plain the rudimentary fifth anal plate of some specimens of Parasalenia seen 

 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 Cladosaleuia, but which I have since found 

 to be only small Parasaleniae. In all the Echinometrae known to me the anal 

 system 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 PI. 

 III''.f. 1, 2, of the Revision of the Echini. I cannot agree with Troschel in 

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

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

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

 connection whatever with the abactinal system, and 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 fiimily thus far 

 where the position of the ocular plates seems of generic value is in the 

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

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

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

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

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

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

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

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

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

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

 plates. 



* TuoscnEl., Die Faniilie tier Erliinocidaridon, 1873, Wiog. Airliiv. 

 t A. Agassiz, Embryology of EchiuodLTiiis, Mt'in. Am. Acad., 18G3. 



