364 PROCEEDTNGS OF THE NATIONAL MUSEUM. vol. xxxvi. 



calyx, and might very well result in causing the basals in the 

 adult to occupy a position under alternate radials. The ap- 

 pearance of the basal rays in Decametrocrinus between the radials 

 instead of under alternate ones is a difference of considerable mor- 

 phological importance, suggesting that the young are of quite differ- 

 ent structure. DecametrocHnus is a meristic variation from Penta- 

 metrocrinus ; Thaumatoerinus^ probably the young of Pentametro- 

 crimts, has interradial plates of equal size in all the interradial areas ; 

 now if the young of Decametrocrimis were of the Thaumatocj^nus 

 type with five equal interradials which, during growth, were shoved 

 out from between the radials at an equal rate, the basal rays in the 

 adult would maintain exactly the same relation to the radials as the 

 basals did to the radials in the young, instead of being twisted about 

 into a semiradial position as in Promacliocrlnus, Thus a compari- 

 son of adults of the ten-rayed genera of the Pentametrocrinidoe and 

 Antedonidse leads to the same conclusions as a comparison of Thau- 

 matocrinus with the Antedon larva, namely, that T haumatocrinus is 

 the young of Pentametrocrinus. 



By this reasoning I had sometime ago reached the conclusion that 

 ThamnatocAnus was very close to PentaTYietrocrinus^ and probably 

 the young of it, and I therefore placed it next to Pentametrocrinus 

 in the family Pentametrocrinida?. Here the matter rested, for noth- 

 ing further could be done without additional facts to prove or dis- 

 prove the results attained by purely speculative processes. 



The young example of this new comatulid is so like Thaumato- 

 crinus in certain w^ays as to convince me that I was right in my 

 tentative treatment of that genus. It represents, however, a more 

 advanced stage ; the five large, strong orals are present as in T. reno- 

 vatus, surrounded by small irregular plates; the basals are not evi- 

 dent externally; the radials are in lateral contact, and just above 

 their apposed lateral edges in the angles of the calyx are five large 

 interradials which appear to have been recently thrust forward from 

 between them, but which are not yet undergoing the process of re- 

 sorption. In the large and persistent orals and interradials (though 

 displaced) this young specimen resembles Thamnatocr'inus, though 

 it is true that it differs from it in the approximation of the radials and 

 in the absence of external basals ; but there can, I think, be no reason- 

 able doubt that these differences are merely the result of its greater 

 development. The arm structure, so far as it is elaborated, resembles 

 that of the adult. 



This new form is undoubtedly referable to the Comasterida?, as 

 evidenced by the characteristic pinnules with short joints, coarsely 

 spinous on their distal ends, finely spinous dorsally, the comb on the 

 terminal portion of the first pair, the coarsely spinous overlap of the 

 brachials, and their spinous dorsal surface. This appears to out- 



