236 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



by a progressive decrease in the height, resulting from a planing off, by resorption, 

 of (he dorsal ]w]o; tliis results, o\\4ng to the hemispherical outUnc in a progressive 

 hroaih'iiing of the dorsal j)ole, wliich at the same time becomes flatter, and in tiie 

 eUmination, one by one of the rows of cirrus sockets, so that the centrodorsal finaUv 

 becomes a broad flat disk witli a single, often more or less deficient, irregular mar- 

 ginal row of cirrus sockets; the process continuing furtiier, tliis (hsk becomes thimier, 

 the cirri, one by one droj) off, the sockets close up, and the disk then begins to 

 decrease in diameter, finally retreating witlun the circlet of ra<hals and sinking so 

 that the doi-sal surface of the rachals and of the centrodoi-sal both rest in a common 

 plane (figs. 152, 154-156, p. 221). In extreme cases the radial margin of tlie disk 

 is resorbed and becomes more and more concave, the interradial portion always 

 reacliing to the ends of the basal rays, until a small tliin sliarply stellate plate 

 results (figs. 157-159, p. 221). 



The suppression of the cirri follows exactly the same lines as their development; 

 they first dlsajipear one by one from the midradial region of the centrodorsal (fig. 

 531, pi. 2); an incipient stage of this process is frequently noticed m certain of the 

 Thalassomctridaj (compare figs. 196 and 198, p. 2.37); then the whole of the radial 

 region becomes affected, so that the cnri arc reduced to the interradial portions, 

 occurring, singly or in pairs, just beneath the interradial angles of the calyx; this 

 condition is permanently retained in the adult of Comatula purpurea (fig. 79, p. 

 132), and is often noticed, as an mdividual variation, iii many of the species m 

 which the cirri are normally lost in the adult, as for instance, in Comanihina schlegelii 

 and in Coniaster leUi; at last these interra<lial cirri begin to drop away, so that only 

 one cuTUS is left in each interradial angle, and finally aU the cirri are discarded. 



P. H. Carpenter notes that the ventral surface of the centi'odorsal of Comanthus 

 parmdrra is 10-sided or nearly so (figs. 243-245, and 247-249, p. 251), and is not 

 marked b}' shallow radial depressions like those seen on the ventral surface of the 

 centrodorsal of Antedon (figs. 280, 281, 283, p. 261, and 593, pi. 15). The radial 

 areas lisc very slightly from their peripheral to their central margins, and are 

 marke<l by various mdistinct ridges and furrows. Their sides rise towards the 

 five interradial elevations wlxich, though not very much raised above the general 

 surface of the plate, are nevertheless very distinct; for they are wide and marked 

 by shallow grooves which occupy the greater part of their width, so that the smi- 

 ple ridge, as seen in Ilathrometra (fig. 290, p. 262) and Leptometra (fig. 287, p. 262), 

 is here represented by the two sides of the groove which is cut out along its median 

 line. In Antedon these sides meet at a very short distance from the central end of 

 the groove, so as to obliterate it (fig. 285, pi. 261). In Comanthus parvidrra, how- 

 ever, they approach one another very gradually, and only just meet ^Wthin the 

 margin of the plate (figs. 243-245, and 247-249, p. 251); but the ridge forme«l 

 by their fusion does not end here as in Antedon, for it is continued a short dis- 

 tance beyond the general surface of the plate so as to appear as a short jirocess 

 extemling outwards from the angle between two sides of its external j)ent agonal 

 margin. Consequently these five short processes appear on the dorsal aspect of 

 the plate, prolongmg its angles outward. The grooves which are thus cut out 

 along the median line vi the interradial elevations on the ventral surface of the 



