Bd. VI: 4) 
THE ECHINOIDEA. 
29 
Fig. 7. Spicules 
from the tubefeet 
of Arbacia Dufres- 
nii. I25 /i. 
spicules in the upper part of the tubefeet; they are more or less irregular, with some 
perforations in the broader middle part (Fig. 7). 
This species was taken by the Expedition only at Station 2, the coast of North 
Argentina (37 0 50' Lat. S. 56° 11' Long. W. 100 m.); 4 large, high specimens. 
From the Fuegian Expedition there are several large and small specimens from 
Bahia inutil, 1 — 11 fathoms, 23 /i 1896, Ultima Esperanza, 7— 10 fathoms, stones, algæ, 
3 specimens, Puerto Madryn, 5 fathoms, sand-clay bottom, IO /xi 1 §95 7 5 specimens. 
The question whether Arbacia alternans (TROSCHEL) is only a 
synonym of A. Dufresnii or represents a distinct species, has 
been very much discussed and a general agreement has not been 
reached as yet. LovÉN decidedly regards A. alternans as distinct, 
as does also STUDER. Likewise it is named as a separate species 
by MEISSNER in Bronn’s »Classen u. Ordnungen d. Tierreichs», 
Echinoidea p. 1360. De Loriol (Op. cit.) maintains that in the 
characters of the tuberculation of the abactinal side — the character 
upon which alternans is mainly established — no constant difference 
can be found, but he thinks the characters in the pedicellariæ pointed 
out by STUDER »bien plus que l’arrangement des tubercules doivent 
engager à maintenir VA. alternans comme espèce distincte». AGASSIZ 
(»Challenger» Echinoidea) evidently regards alternans as synonymous with Dufresnii 
since he does not mention the former in his list of the known species of recent 
Echini (p. 209). 
In the latest, very important, contribution to the knowledge of this group of 
Echinoids, viz. AGASSIZ & Clark’s »Hawaiian and other Pacific Echini. The Arba- 
ciadæ etc.» both alternans and africana TROSCHEL are regarded as synonyms 
of Dufresnii (p. 68), with the reservation that »it is possible that further material 
from the west coast of Africa will make the recognition of the species africana 
TROSCHEL desirable, and there is also a possibility that alternans TROSCHEL pay 
ultimately be separable from Dufresnii Bl.» (p. 67). — There is thus evidently upon 
the whole a tendency towards maintaining alternans as a distinct species. 
From the Berlin Museum I have received the type specimens of TroSCHEL’s 
alternans for renewed examination. The largest of them is represented in PL V. 
Figs. 6 — 7. Further I have received, through the kindness of Professor ThÉEL, a 
specimen in the collection of the Stockholm Museum, identified by LovÉN as alter- 
nans. A very careful examination and direct comparison of these specimens with 
the specimens of A. Dufresnii in hand has convinced me that it is impossible to 
maintain alternans as a distinct species. 
The alternation of the primary abactinal interambulacral tubercles, upon which 
character alternans was mainly distinguished, is decidedly without value as a specific 
