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

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

 wliich they liel]) to support. Five of them, those in the interradial angles, are 

 often considerably' larger than the rest, and may bo 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 othcre of tlie same species." 



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

 genera in wliich 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 Httle or no lip 

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

 order Macroi)hreata, comprises the families Antedonidae, Atelecrinidje and Pcn- 

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

 famihes Comasteridse, Zygometridas, Himerometridse, Stephanometridfe, Maria- 

 metridae, Colobometridae, Tropiometridse, Calometridse, Thalassometridse, and 

 Charitometridse. 



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

 a glance at the ca\-ity of the centrodorsal; but caution must alwaj"s 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 oligojihreate 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 Comastcridai are remarkable for the great diversity in the size of the centro- 

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

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

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

 dorsal i)ole, 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 ComatuJa micraster, Capillaster macrohrachius, 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 shghtly below it, from which it is separated by 

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

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

 arc found; but the centrodorsal in the Comasteridae 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 hemisjiherical centrodorsal of Comanthus 

 hennetti or C. pinguis and the small stellate disk of Cotnastcr ttjpica is effected simply 



