BIHANG TILL K. sv. VET.-AKAD. HANDL. BAND 13. AFD. lY. N:0 5. 81 



MOULINS^) was inclined to the same opinion, adopted also by 

 Al. Agassiz^). 



The word »saepe>^ of the description indicates that Lin- 

 N^us had before him several specimens, and there are in the 

 Drottningholm collection five of a species of Arbacia, all 

 denuded. I shall call them a, b, c, d, e. They occupy four 

 boxes. Of these two, containing a and e, are marked with 

 the usual >Mus. Gust. Ad. in Thunbergs hand, and a is pro- 

 vided with a printed label: Echinus Lixula. The two other 

 boxes, one for h, another for c and d, have each the Swartz- 

 ian label alone. The authenticity of these specimens is thus 

 fully established. They answer completely to the Linnean de- 

 scription of his E. Lixixla, and are without doubt its prototypes. 

 The specimen e is not in a sufficiently good state for being 

 taken into account, and has been omitted. 



Two species, dispersed under Nos 18 and 24 among the 

 Echini of Lamarck^) were recognised by Blainville^) as for- 

 ming, togother with some others known to him, a natural 

 group, the division A of his highly meritorious monograph. 

 Ten years låter it had a generical name given it by Gray^): 

 Arbacia, and a few months after another by Desmoulins^): 

 Echinocidaris. Its limits all the time remained the same until 

 lately Duncan and Släden^) on good groiinds removed the 

 well-known Echinus niger of Molina, which is to be the type 

 of a genus of its own, bearing the name Echinocidaris, all but 

 coeval with Arbacia, and under which it was placed as the 

 leading species by Desmoulins^), though under the wrong 

 appellation of E. pustulosa. The x\garites and Tetrapyga of 

 Agassiz"), the Py gömma of Trosghel'"), are posterior. 



Thus limited the genus Arbacia presents the following 

 characters, such namely as are generally used in comparino- 

 and determining denuded specimens. 



') Etudes, p. 307. 



^) Revision, p. 92. 



3) An. s. vert., III, p. 47, 49; 1816. 



*) Diet. Se. Nat., XXXVII, p. 75; 1825. 



') Proc. Zool. Soc., III, p. 57; 1835, April 28. ~ Reprinted: Phil. 

 Mag., VII, Nuniber XL, October 1835, p. 329. 



^) Etudes s. 1. Echinides, p. 34. Actes Soc. Lin. Bordeaux, VII, 4; 

 1835, August 15. 



') Journ. Lin. Soc, Zool., XIX, p. 53. 



8) 1. c. p. 304, Actes, IX. 1837; XXVII, pl. 11, fig. 7, 8. 



») Monogr. d'Echinod., IL Scutellcs. Introd. p. 7: 1841. 



'O) Archiv f. Naturg., XXXVIII, p. 293, 1872: XXXIX, p. 308; 1873. 



