MORPHOLOGICAL PART. 71 



Upon commencing the present work, we had an interchange of views 

 with Dr. Carpenter as to the best plan to be pursued respecting these plates, 

 especially as to whether it would not be better to change their terms, which 

 we considered meaningless from a morphological point of view, for others 

 more appropriate. We proposed to restrict the term "radials" to the first 

 ring of plates of the rays, and apply to the succeeding ones the terms " pri- 

 mary," "secondary" and ^^ tertiary brachials," according to their rank, in 

 all cases, whether the plates were free or not. He replied that he appre- 

 ciated the great advantages of the proposed nomenclature, and was willing 

 to adopt it in principle; but he thought the terms ^^ primary," "secondary," 

 and "tertiary brachials" were too long and cumbrous, and he proposed in- 

 stead of them the terms "costals," "distichals" and "palmars," respectively; 

 which we finally agreed to accept. Carpenter announced our agreement 

 in a paper " On certain Points in the Anatomical Nomenclature of 

 Echinoderms." ^ 



In the earlier Inadunata and Articulata, — not in the Camerata so far as 

 observed, — the radials are frequently compound, i. e., constructed of two 

 segments or parts, which are closely united by a horizontal suture, and in 

 the organization of the Crinoid count as one plate.f Among the Fistulata, 

 compound radials are found in the Heterocrinidse, Anomalocrinidge, Hybocri- 

 nidae and Dendrocrinid^. In some of these families they are restricted to 

 the right posterior ray, in others they occur also in other rays. We know of 

 no case in which the compound structure extends to all five radials, at least 

 two of the plates being always simple. Among the Heterocrinidse, as a rule, 

 the right posterior, and the right and left antero-lateral radials, are compound, 

 — exceptionally the anterior one in place of the left antero-lateral; — while 

 Anomahcrimis and Ohioerimis have but two, and locrinus, Merocrinns^ Hyhocriniis 

 and Hoplocrinus only one. It is further worthy of note, that when there are 

 several compound radials, the corresponding parts are of nearly equal size ; 

 while the segments vary considerably among themselves in the different 

 genera. In Heterocriniis, Ohioerimis, locriniis, Merocrinus, Il^/hocrimis and ffyho- 

 ci/stis, the lower portion, — the so-called inferradials, — are considerably 

 larger than the upper or superradials. In Anomahcrimis and Dendrocrims 

 the two plates are of nearly equal size; while m Edenocrimis {Heterocrinus) 

 simplex the npper ones are three or four times as large as the lower. In 



* Ann. and Mag. Nat. Hist., Ser. 6, Yol. YL, pp. 11-18. 



t We gave a full description of these plates in a paper " On the Perisomic Plates," Proceed. Acad. Nat. 

 Sci. Phila., October, 1890, pp. 378 and 379. 



