240 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



or at most two rows of cirri, represent an advanced stage of conmsterid centrodorsal 

 development (though the arm structure in these two genera is much less specialized 

 than in Comantkus), while the stellate centrodorsals of CorrMster typica or of Capil- 

 laster macrobrachius are of the most specialized type. 



It is a curious fact, though one finding innumerable parallels, that in all of the 

 genera of the Comasteridse the centrodorsal starts, so far as we know, from exactly 

 the same condition in the young, and develops along exactly the same lines; in 

 Comanihus all the stages are found in the adults of the various species, but in the 

 other genera the sum of the species taken together covers only a comparatively 

 small part of the entire developmental line. 



If we take the line of development of the comasterid centrodorsal and divide 

 it into four parts, marking the division points A, B, C, and D, A being the Comanihus 

 lennettitype (figs. 171, 174, p. 231) (under which, in effect, all the very small species 

 such as Comatilia iridometriformis are included, as would be expected); D the small 

 stellate Corn-aster typica type (figs. 157-159, p. 221), B (figs. 146-148, p. 220) and C 

 (figs. 160, 161, p. 223, and 163, p. 225) intermediates, we find that Comactinia, Lep- 

 tonemaster, Neocomatella, Comissia and Nemaster all fall between B and C; Cominia 

 falls in B; Comatella extends from A to C; Comanthus from A to D; Capillaster from 

 B to D; and Comatula and Comaster from C to D. Palseocomatella is essentially 

 like Neocomatella, though it exhibits a tendency toward a columnar arrangement of 

 the cirrus sockets. 



It is interesting to note that, except for the very small species of Comatilia 

 and Microcomatula, which are scarcely to be considered in this connection, the 

 West Indian comasterids and the comasterids occurring on the Atlantic coasts of 

 Africa are restricted in regard to the development of the centrodorsal to the interval 

 B-C, whereas those of the central East Indian region and of the more northern 

 portions of Australia range from A to D with the emphasis, in Australia, on the D; 

 of other regions, the northwest and southeast African comasterids range only 

 between B and C like the West Indian, while the southern Japanese range from 

 AtoC. 



It is evident from the tabulation given above that the comasterid genera which 

 show the most specialization in other characters have also the most specialization 

 in their centrodorsals, and also that extreme specialization, either in the direction 

 of a retention of a larval type of centrodorsal, or of very great reduction in the 

 size of that plate, is confined to the areas where extreme specialization in other 

 characters occurs. 



In the Innatantes the central plate is not comparable to the centrodorsal of 

 the other comatulids (figs. 565, 572, pi. 7) ; I believe it to be the homologue of the 

 terminal stem plate plus all the columnals of the other comatulids. I am led to this 

 belief from the following circumstances: It lies in the body wall flush with the infra- 

 basals, and therefore can not be a columnal, for in all stalked crinoids the topmost 

 columnal supports more or less of the lower margin of the basals or of the under- 

 basals; this is a mechanical necessity, as otherwise the weight of all the calcareous 

 structures would have to be taken up by the soft interior structures immediately 

 above the stem, and by the sutures between the topmost columnal and the 



