SHALLOW-WATER STARFISHES 281 



In some families the plates are reticulated ossicles, smooth, or 

 bearing large spines or tubercles as in Oreasteridae. In other cases 

 (Porania, etc.) a thick skin may cover and obscure the plates. 



The interactinal plates may be few in stellate forms or many in the 

 pentagonal forms. In the latter cases they are usually arranged in 

 definite rows, most often parallel with the adambulacral plates, the 

 latest formed ones being situated next the median, interradial, mar- 

 ginal plates. 



The interactinal plates are most commonly tesselated and granu- 

 lated, or else in the form of pseudopaxillae or protopaxillae ; but they 

 are sometimes spinose. They may be more or less imbricated by their 

 edges or lobes. 



Adambulacral plates are not compressed, rather large, usually 

 quadrangular, and commonly bear numerous small spines, various in 

 arrangement. 



Ambulacral plates are rather large, not compressed nor crowded. 

 Ambulacra! feet are in two rows, with terminal suckers in Valvu- 

 losa ; but in the Paxillosa large and pointed, or without suckers. 



A median, dorsal, nephridial pore or pseudanus is generally pres- 

 ent, but often absent. Madreporic plate various. Dentary or oral 

 plates are often large, with numerous oral spines. 



fascicles. These are highly developed in most species of Astropecten and 

 Luidia. 



" Spinofiaxilla 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 Luidia, Pontaster, etc. 



" ParopaxiUcF are lower and broader, 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 sum- 

 mit is covered with small, short, differentiated spinules, like those of true 

 paxillae. Those on the dorsal surface of Plutonaster are examples. They 

 sometimes bear a central spine. 



" Protopaxilla are similar, but less elevated, convex ossicles or plates, 

 covered with round or angular granules, with the marginal series dif- 

 ferentiated and more or less covering the grooves between the plates. As 

 m the preceding, there may be a central spine in some cases. This form also 

 occurs on Plutonaster, and on many species of Goniasteridse. The transition 

 from this last kind to simple, uniformly granulated plates is easy, when the 

 grooves between the plates become obsolete. 



" Pseudopaxilla are articulated plates with a flattened, usually lobed, and 

 often overlapping base, which bear a group of slender, fascicled or divergent 

 spinules on the more or less raised central or subcentral area or boss. These 

 have no differentiated marginal series of spinules. This form is seen in 

 Solaster, Henricia, etc." 



