1884.] SPECIES OF OREASTER. 0*3 



been necessary to make a close inquiry into the structural characters 

 of an Oreaster (e. g. O. nodosus), Nidorellia armaia, and Paulia 

 horrida. 



One of the leading distinctions of the family Goniasteridse is the 

 solidity and strength of those ossicles which Gaudry has called inter- 

 ambulacral ; among the members of the family in which the " tergal 

 ossicles" are reticulated, Oreaster, as exhibited by O. nodosus, 

 stands out conspicuously ; and we have, therefore, in the first place, 

 to speak of the genus as a Goniasterid in which the tergal ossicles 

 are always so reticulated that pore-areas more or less extensive are 

 observable among them. 



So far the definition applies to Nidorellia as much as to Oreaster; 

 but it could only be applied to Paulia by saying that the reticular 

 character is obscured or destroyed by the investing granulation. 



Secondly, among the Goniasterids, we find that the two outer- 

 most rows of interambulacral plates take on a special development, 

 and form a well-defined margin to the side of the Starfish. These 

 plates may or may not be armed with spinous or tubercular pro- 

 minences, and they may or may not have smaller plates intercalated 

 among them. In Oreaster such plates are never intercalated, the 

 investing granules do not project from their side so as to separate the 

 ossicles from one another, nor are they ever limited to the base, but 

 always pass some way up the sides of the spines or tubercles, when 

 such are developed. 



This statement is as true of Nidorellia armata as of 0. nodosus, 

 but it does not apply to Paulia. 



It would seem to follow, therefore, that Liitken was justified in 

 regarding N. armata as an Oreaster, and that the return by Perrier 

 to Gray's original view was a retrograde step. For the present, 

 the characters and relations of Paulia may be put on one side as 

 well as the exact position of O. ohtusatus, which must be removed 

 from its original generic position in consequence of the presence of 

 intercalated smaller plates among the marginal osssicles. 



The species of this genus fall into two well-marked groups, in one 

 of which large tubercles may be, but spines are never, developed ; in 

 the other a certain number of spines are constantly present,, and they 

 may be both large and numerous. Intermediate to these stand 

 such as have the lophial spines reduced or absent, but others deve- 

 loped on various parts of the skeleton. The two chief divisions 

 may be distinguished as those of the inermes and armati ; and in 

 each the species may be separated according to the number of rows 

 of spines which are typically and most generally developed on the 

 adambulacral plates. In no case do we find a Monacanthid arrange- 

 ment. 



The following series commences with the unarmed, and ends with 

 the triplacauthid well-armed forms. 



