84 NATURAL HISTORY BULLETIN 



This order includes starfishes varying in form from pentagonal 

 to stellate, with long rays. They have two rows of large and 

 thick marginal plates (rarely but one distinct row, as in Luidia), 

 which are usually in contact and form a thick, nearly perpendic- 

 ular margin. They are nearly always larger and thicker than 

 the dorsal and actinal plates. Papular pores are nearly always 

 confined to the upper surface and are usually placed singly, 

 often in special areas. Dorsal skeleton various. The dorsal sur- 

 face may be closely covered with angular tesselated plates, 

 which may be naked, granulated, or covered with a smooth skin, 

 or they may take the form of pseudopaxillfe, protopaxillae or true 

 paxillae especially in the Paxillosa. 



The various forms of paxilliform plates as distinguished in 

 1894, by the writer, are as follows : 



True paxillce are columnar or hour-glass-shaped ossicles, usu- 

 ally with isolated, circular or lobed bases, which bear at summit 

 a group of small spinules, of which the marginal series are dif- 

 ferent from the rest and divergent, so as to cover the interven- 

 ing spaces between the spines, thus forming fascioles or channels 

 for the circulation of water to the papulse. These are highly de- 

 veloped in Astropecten and Luidia. Those called spinopaxilke 

 are of the same general structure, but the center of the summit 

 is occupied by a distinct spine, or by more than one. Such forms 

 occur on some species of Luidia, Pontaster, etc. 



Parapaxillce are rounded or stellate ossicles, or angular plates, 

 with a raised central portion, tabulate, truncate, or like a low 

 cone. They may be either isolated or articulated by their bases. 

 The summit is covered with small, differentiated spinules, like 

 those of true paxillae. Those on the dorsal surface of Lophaster, 

 Solaster, and Mediaster are examples. They sometimes bear a 

 central spine. 



Protopaxilloi are similar, but less elevated, convex ossicles or 

 plates, covered with round or angular granules, with the mar- 

 ginal series differentiated and more or less covering the grooves 

 between the plates. As in the preceding, there may be a central 

 spine in some cases. This form occurs on Cheiraster, many 

 sipecies of Go7iiasteridce, etc. The transition from this last kind 

 to simple, uniformly granulated plates is easy, when the grooves 

 between the plates become obsolete. 



