234 BULLETIN- 82, UNITED STATES NATIONAL MUSEUM. 



interior cavity are nearly or quite flat, or regularly curved, but in many comatulids 

 they are marked by strong ribs alternating in position with the columns of per- 

 forations through which pass the cirrus canals, " the lower ends of which are more 

 or less distinctly visible through the central opening, projecting beneath its lip, 

 which they help to support. Five of them, those hi the interradial angles, are 

 often considerably larger than the rest, and may be the only ones visible. In other 

 cases, however, both these and numerous smaller intermediate ribs are visible 

 through the central opening. These ribs are much more distinct in some individ- 

 uals than in others of the same species." 



The recent comatuhds are at once divisible into two great classes, one including 

 genera in which the central cavity of the centrodorsal is typically very large and 

 deep with usually a prominent ventral lip (figs. 66, p. 93, and 286-291, p. 262), the 

 other containing genera in which it is very small and shallow, with little or no Up 

 (figs. 68, p. 93, and 250-255, p. 253). The first division, constituting the sub- 

 order Macrophreata, comprises the families Antedonidae, Atelecrinidae and Pen- 

 tametrocrinidae, and the latter, known as the suborder Oligophreata, includes the 

 families Comasteridae, Zygometridae, Himerometridas, Stephanometridse, Maria- 

 metridae, Colobometridse, Tropionietridas, Calometridse, Thalassometridas, and 

 Charitometridae. 



Usually species may be referred at once to one or other of these two groups by 

 a glance at the cavity of the centrodorsal; but caution must always be used, for 

 very large specimens of some macrophreate forms, and certain large species, in- 

 crease the outer walls of the centrodorsal faster than they excavate the central 

 cavity, and hence approach in appearance the oligophreate forms (figs. 67, p. 93, 

 and 297, p. 263), while small and immature oligophreate specimens, or the less 

 specialized species, may at first glance appear to be macrophreate (fig. 235, p. 249). 

 The Comasteridse are remarkable for the great diversity in the size of the centro- 

 dorsal, even within the limits of a single genus, sometimes even within the compass 

 of a single species. In some forms, as in Comanfhus bennetti or C. pinguis (figs. 171 

 174, p. 231), it is very large and hemispherical with a small strongly concave 

 dorsal pole, and bears several more or less irregular alternating rows of cirrus sockets 

 which are large and crowded, resembling somewhat the centrodorsal of some of the 

 large species of Heliometra or Florometra (figs. 225, 226, p. 243); in other species, 

 as in Comatula micraster, CapiUaster macrobrachius, Comaster typica, and Comantheria 

 polycnemis, it is reduced to a small pentagonal or stellate plate, devoid of the least 

 trace of cirrus sockets and countersunk so that its flat dorsal surface is even with 

 that of the radial circlet or even slightly below it, from which it is separated by 

 deep and narrow clefts, bridged over by the ends of the basal rays (figs. 162, p. 

 223, 164, p. 227, and 166-170, p. 229). All gradations between the two extremes 

 are found; but the centrodorsal in the Comasteridse is exclusively of some type 

 between these two extremes and never becomes conical or columnar as is frequently 

 the case in other families, nor are the cirri (except in a single aberrant genus) ever 

 arranged in columns. 



The transition between the large hemispherical centrodorsal of Comanthus 

 bennetti or C. pinguis and the small stellate disk of Comaster typica is effected simply 



