96 PKOCEEDIKGS OF THE ACADEMY OF [1887. 



central plate, surrounded by others, either six or seven. Even the 

 original of Etheridge and Carpenter's figures 11 and 12 on PI. XV, 

 in our collection, upon the api^lication of coloring matter to bring 

 out the sutures, proves to have a quite regular arrangement of the 

 summit plates, which the artist who drew the figures failed to detect. 

 There are certainly not "five small plates," but a central plate 

 surrounded by six proximals. 



According to our observation, therefore, of the best preserved 

 material known, the summit of 0. stelliformis does not represent the 

 "simplest form," "just as in Stephanocrums." 



There remains only to consider Grcmatocrinus Norwoodi and 

 Schizoblastus Sayi, as to both of which we remark that there is often 

 presented much apparent irregularity and variability in the 

 arrangement of their summit plates. But we find that this is due to 

 the enci'oaciiment of the covering plates, which sometimes largely 

 overlap them, as is well shown by Ether. Carpenter's PL VII, figs. 

 11 and 13. But in natural internal casts, in which we have the 

 impressions of the inner surface of the jjlates, they appear larger 

 and much more regularly arranged. Among a lai'ge number of 

 specimens we have failed to find a single example of a summit 

 closed by only five plates; while in a large proportion of them we 

 distinguished clearly a central plate surrounded by six or more 

 proximals. Whatever variations, however, of form and arrangement 

 of summit plates may be found to exist in these two species, we feel 

 warranted in asserting that the "simplest form" is not one of them. 



On page 71, (Catal, of the Blastoidea), Messrs. Etheridge and 

 Carpenter say that "in 1877, Wachsmuth pointed out that a 

 definite arrangement of plates is more or less traceable in the vault 

 of many Palaeocrinoids. There is a single central plate, wdth five 

 or, more frequently, six others disposed interradially round it." 

 It would naturally be inferred from this remark and the context, 

 not only ihixtjive proximals around a central plate is one recognized 

 condition in the vault of many Palaeocrinoids, but also that 

 Wachsmuth had so expressed himself: whereas the fact is he said 

 nothing of the kind in the paper cited, but on tlie contrary spoke 

 only of "7 apical plates" a central, four large, and two small ones; 

 and this number, or a greater, has been insisted on by us as being 

 the almost universal rule. 



In seeking for a transition or variation in the summit plates of 

 Palaeocrinoids, comparable to that which they assume to exist 



