1884.] SPECIES OF OREASTER. 59 



best adapted for arriving at some clearer ideas as to the relations of 

 the species among themselves, and the history of an ancient generic 

 group. 



First of all, we may well expect some differences in external 

 appearance, in the relations of the greater to the lesser radii, and in 

 the width and proportions of the arms, the moment we know tliat 

 specimens may attain to a spread of 400 millims. or more, or attain 

 to a height of 120 millims. ; while, however, we shall find growth- 

 differences in some, we shall in others, such as O. nodulosus, be 

 struck rather by the constancy of proportions in the post-larval 

 stfiges of devolopment. 



Our experience of other long-armed forms, such as Linckia or 

 Oj)hidiaster, might lead us to ask. Does Oreaster, like these genera, 

 tend to lose its arms, and does it, like them, reproduce itself asexually, 

 or exhibit any other mode of heteractinism ? Heteractinic conditions 

 are exceedingly rare among Oreasters, and it follows therefore that 

 the dangers to which the species are exposed are slight, its skeletal 

 structures are very strong, or its power of active or passive defence 

 very great. 



As to the danger we know but little ; as to the skeletal struc- 

 ture, we know that it is eminently reticulated on the upper 

 surface ; and, now, as to the organs of defence, we know that many 

 of the species are well provided with marginal or dorsal spines of 

 considerable length, and that, in some cases, the jjroper ventral 

 plates are very spinous. 



To a certain extent these spines present us with very definite 

 characters. We can, for example, always safely discriminate between 

 O. lincJii and 0. nodosus, by examining the free ends of the arms, 

 the sides of which in the former are constantly, and in the latter 

 are never, provided with outstanding spines. So, again, the species 

 described by Perrier under the name of 0. alveolatus may, as it 

 seems, be certainly separated from O. lincJci, owing to the fact that 

 the infero-marginal plates bear well or fairly developed spines. 



For the purposes of this investigation we shall, perhaps, do well 

 to study attentively one of the species of the genus in which the 

 spinous armature is well developed — 0. arfnatus : three specimens, in 

 which R is respectively equal to 23-5, 37, and 85 millims., have at the 

 proximal end of the middle line of each arm a spine measuring I, 3, 

 and 14 millim. respectively. In (a) the marginal plates rarely 

 exhibit any break in their regular granulation ; when they do so, 

 we find a naked pajjilla just projecting beyond the level of the 

 granular investment ; no spines are developed in the spaces between 

 the middle lines of each ray; in the centre of the disk is a spine 

 which is about equal in size to those which mark the end of the 

 arms ; the other spines along the middle line are nothing more than 

 mere papilliform processes. On the ventral plates spines are 

 developed indeed, but they are as yet only rounded projections 

 which are just beginning to be distinguishable from the investing 

 granules of the ossicles which bear them. 



In the next specimen (/3) the spines of such marginal plates as 



