ECHUrODEEMATA. 127 



the ossicles have diminished iu proportional thickness ■while in- 

 creasing in length, and so having lost their earlier stoutness, now 

 require externail aid ; and (c) a iinal condition, in which equilibrium 

 is again established, the ossicles themselves being again stouter, — or 

 whether -we have to do with two sets of variations from a common 

 stock, due to some slight diflferences in the environment. But this 

 is a complex and diificult question, which can only be satisfactorily 

 answered by one who has at hand the living wealth of the Aus- 

 tralian seas. 



Port Curtis ; Fitzroy Island ; Albany Island ; Port Denison, 

 8-12 fms. 



10. Linckia, sp, 



A very small specimen, from West Island, Torres Strait, presents 

 that interesting peculiarity of three smaller and three larger arms, 

 hinting thereby at a division of the disk instead of gemmation from 

 a single arm; it is the smallest heteractiuic Linekia I have seen, the 

 longest arm measuring only 6 millim. 



11. Antheuea flavescens. 

 Perri'er, p. 276. 



Port Jackson, 0-5 fms. 



The Museum collection contains also specimens of this species 

 from Fremantle, W. Australia. 



12. Oreaster gracilis. 



Pentaceros gracilis, Perrier, p. 246. 



Oreaster gracilia, IMtkm, Vid. Med. 1871, pp. 260, 261. 



A selected series of five specimens, all from Port Denison (4 friis.), 

 exhibit remarkably well the great change which occurs in this 

 species, even after a considerable size has been attained. The 

 smallest, wliich has R equal to 93-5 miUim., has five tubercles, 

 about 10 millim. high, one at the central end of each median row 

 of tubercles : in another, with E equal to 108 millim., the 

 tubercles, though a little stronger, are no higher and are here only 

 to be found on three of the angles of the disk ; this specimen, like 

 another which is a little larger, has the lateral and marginal spines 

 very well developed : two specimens, with E, equal to 118 and 

 140 millim. respectively, have these spines less well developed, and 

 the diminution is th.e more marked in the larger specimen; the 

 smaller has one large central tubercle, and the larger has each of 

 the whole set of five reduced to growths which are hardly larger 

 than the other spines on the disk. 



A specimen with six arms, one of which has been injured and 

 has commenced to grow again, was collected at Port MoUe. 



