22 P. HKEBERT CAEPENTER. 



a dorsocentral plate as well as under-basals, but the same is the 

 case with some Ophiurids, as I have pointed out above (PI. I, 

 figs. 1, 2, 12); and with many Asterids as shown by Sladen (PI. I, 

 figs. 14, 16). Wachsmuth and Springer^ seem to have felt 

 this difficulty ; for they do not, as Loveu and Agassiz did, 

 regard the plate within the ring of under-basals as a dorso- 

 central ; but they are inclined to think that it represents the 

 column of Crinoids generally. This supposition, however, is 

 not necessary when the condition of the apical system of 

 ' Stellerids is taken into consideration, as is shown both in the 

 preceding and in the following pages. 



Postscript. — Since the above was written, I have received, 

 by the kindness of Prof. Agassiz, a copy of the beautiful Atlas 

 of plates illustrative of the embryology of Echinoderms.^ For- 

 tunate, indeed, is the student who has access to this most useful 

 work. But, without in any way desiring to be hypercritical, I 

 I could wish that a little more uniformity and precision of no- 

 I menclature had been adopted for the plates of the Apical 

 I System. 



\ The cirrus-bearing top stem-joint of Comatula receives its 



proper name, centrodorsal^ as used by Sir Wyville Thomson 

 and Dr. Carpenter. But this name is also applied to the cen- 

 tral plate in the Apical System of the Stellerids, which is 

 another structure altogether, as I have often stated in the 

 pages of this Journal. Ludwig invariably spoke of it as the 

 " Centrale,*^ while Loven called it the " Dorsocentral,'' as do 

 Sladen and myself. In some copies of Ludwig's figures, how- 

 ever, which appear in the Atlas, it is marked " Dorsocentral," 

 and in others " Centrodorsal." But since the Echinozoa are 

 stemless, and, consequently, have no Ceutrodorsal, I fear that 

 this uncertainty of nomenclature cannot but lead to confusion 

 in the mind of the learner who has been previously studying 

 the earlier plates of Crinoid embryology. 



For the same reason I regret the insertion of the word 

 ' " Revision of the Palaeocrinoidea," part i, p. 22, ' Proc. Acad. Nat. Sci.,' 

 Pliilad., 1879. 



'Mem. Mus. Couid. Zool..' vol. ix, No. 2. 



