56 



THE VOYAGE OF H.M.S. CHALLENGER. 



Station 78. July 10, 1873. Lat. 37° 24' N., long. 25' 13' W. ; 1000 fathoms; 

 globigerina ooze. 



Station 70. June 26, 1873. Lat. 38' 25' N., long. 35' 50' W. ; 1675 fathoms; 

 globigerina ooze. 



Station 73. June 30, 1873. Lat. 38' 30' N., long. 31' 14' W. ; 1000 fathoms; 

 bottom temperature, 3°7 C ; globigerina ooze. 



Aebaciad^. 

 Family Arbaciadse, Gray, 1855, Proc. Zool. Soc. London. 



Arhacia. 

 Arhacia, Gray, 1835, Proc. ZooL Soc. London. 



With the exception of Arbacia nigra, the Challenger collected but few specimens of 

 the genus Ai^hacia, so that no material sufficient to undertake a renewed critical examin- 

 ation of the group was obtained. I have already alluded to some of the objections 

 against the view taken by Troschel ^ of the synonymy of several of the species. The 

 specimens of Arbacia collected by the Challenger at Valparaiso are undoubtedly the 

 common Arhacia nigra ; the specimens of Arhacia dufresnii collected at Nightin- 

 gale Island and other localities may, when compared with the specimens of Arbacia 

 dufresnii collected in the Straits of Magellan by Dr Cunningham, show how far 

 Troschel is justified in considering Arbacia alternam as a distinct species from 

 Arbacia dufresnii. 



Dr Studer, in his list of Echinids from Kerguelen Island, also distinguishes this species 

 from A. dufresnii. Troschel's description is exceedingly minute, but as it is based upon 

 three specimens (one of which is not normal) these differences may after all be only indi- 

 vidual/lifierences. I have abeady, in the Eevision of the Echini and the "Hassler" Echini, 

 called attention to the great variability of the characters of the species of this family 

 drawn from the test, such as the number of tubercles, both primary, secondary, and 

 miliary, the ornamentation of the plates of the test, and the outline. 



Mr Bell ^ has, in a recent number of the Proceedings of the Zoological Society, called 

 attention to the variation in the number of anal plates, and to the tendency these plates 

 show occasionally, in having less than four plates, to retain the conditions of many young 

 Echinids in their youngest stages of having but a single plate covering the anal system; or, 

 when having more than four plates, to pass into the normal condition of all other regular 

 Echinids of having a large number of plates covering the anal system. This variation is, 

 however, by no means so common as he would have us infer from his statistics. The 



1 Wies- Archiv, 1873, Die Familieu Jer Echinocidarideu. 



2 F. J. Bell, Note on the Number of Anal Plates in Echinoddaris, Proc. Zool. Soc. London, May 1879. 



