1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 267 



include mostly genera covered with spinous or granulated plates, pro- 

 topaxillie, or psendopaxilhe.* 



The various kinds of abactinal ossicles pass into each other by vari- 

 ous intermeLliate forms, so that it is impossible to draw any verj^ strong 

 or sharp family lines on this character alone, though the character of 

 the plating may generally be taken as of generic value. 



The existence of definite fascioles of specialized spinules or papilhie 

 on the margins of the plates, so as to form covered channels along their 

 sutures, is evidently a character both of morphological and physiolog- 

 ical importance. The existence of fascioles is correlated with the mode 

 of life. Such forms as have them appear to live more or less buried in 

 soft mud or sand and tlie fascioles are evidently for the purpose of pro- 

 viding a free circulation of water around the whole surface of the body, 

 both to provide for respiration and to keep the surface of the body free 

 from dirt. The paxilliform plates also contribute to both these func- 

 tions. 



The typical Astropectinid;e are among those best provided with 

 fascioles and with the most highly developed forms of paxilhe. They 

 are also those that are eminently dwellers in and beneath mud and 

 sand. Tlie pointed form of the ambulacral feet is correlated with the 

 same habit. 



The family Porcellanasteridse includes Ctenodiscm, Porcellanastery 

 and allied genera, which have similar, but even more specialized, struc- 

 tural adaptations for the same purposes. 



Within the limits of the family Archasterid.e Mr. Sladen made four 



* It seems desirable to have special terms to designate these various forms of dermal 

 ossicles, which are generally included rather indefinitely under the terms paxillae 

 and pseudopaxillaj. As understood by mc, true paxilhv are columnar or hour-glass- 

 shaped ossicles with narrow, usually isolated, bases, which bear at summit a group 

 of small spinules, of which the marginal series are usvially different from the rest 

 and divergent, so as to cover the intervening spaces between the spines. These are 

 highly developed in most species of Astropecten. 



Spinopaxillw 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 Litidia, 

 Pontaster, etc. 



Parapaxilhv. are lower and broader, rounded ossicles, or angular plates with a 

 raised central portion, or like a low column; they may be either isolated or articu- 

 lated by their bases; the summit is covered with small, short, differentiated spinules, 

 much like those of true paxilLe. Those on the dorsal surface of Plntonaster are 

 examples. They sometimes bear a central spine. 



ProiopajrUhi' are similar, but less elevated convex ossicles or plates, covered with 

 round or angular granules, with the marginal series differentiated and more or less 

 covering the grooves between the plates. As in the preceding, there may be a cen- 

 tral spine in some cases. This form occurs on Plulonasier, and on many species of 

 Peutagonasteridae. The transition from this last kind to simple, uniformly granu- 

 lated plates is easy, when the grooves between the plates become obsolete. 



Pseudopaxilhe are plates with flattened, often lobed or branched, and mostly over- 

 lapping bases, which bear a group of slender, fascicled spinules, on the more or less 

 raised central or subcentral area. These have no differentiated marginal series of 

 spinules. This form is well seen in Solasier, Cribrella, etc. 



