Bd. VI: 4) THE ECIIINOIDEA. 29 



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°u' 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, 1896, Ultima Esperanza, 7 — 10 fathoms, stones, algae, 

 ,3 specimens, Puerto Madryn, 5 fathoms, sand-clay bottom, IO / XI 1895; 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. LoVEN 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 Ficr ? Spicules 

 can be found, but he thinks the characters in the pedicellarias pointed from the tubefeet 

 out bv STUDER »bien plus que l'arrangement des tubercules doivent of A ' baLia ^"f 1 lS 

 engager a maintenir X A. alternans comme espece 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- 

 ciadae etc.-> both alternans and afrieana 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 afrieana 

 TROSCHEL desirable, and there is also a possibility that alternans TROSCHEL may 

 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 PI. V. 

 Figs. 6 — 7. Further I have received, through the kindness of Professor Theel, a 

 specimen in the collection of the Stockholm Museum, identified by LOVEN 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 



