THE 8UMMIT-PLA CE8. 73 



vault becomes much more complex, though the orocentral and the proximale (orals, 

 as we regard them) can generally be made out. According to Wachsmuth and 

 Springer 1 , " they are generally larger than the of ber dome-plates, and more prominent, 

 frequently nodose or spiniferous, though in some species they cannot, at least in 

 mature specimens, be readily distinguished from the other dome-plates which nave 

 attained equal size." 



Among the Blastoids we have a similar, though far less extensive, series of varia- 

 tions to that which can be traced among the Palaeocrinoids. The simplest form of 

 summit which occurs in any Blastoid is that presented by Stephanocrinus. The 

 peristome is completely closed by the five triangular plates of the so-called proboscis, 

 which are less distinct in our specimen (PL XIX. fig. 9) than in that figured by 

 Etoemer 2 . Their bases rest against the interradial pieces or deltoids, and they are 

 thus in the same relative position as the large and tubercular proximals of the 

 simple Platycrinus (Fig. VI. B) or of Oulicocrmus, and, as in the latter type, there 

 is no orocentral. But the posterior oral does not appear to be any larger than its 

 fellows, as is the case in Culicocrinus. This is perhaps due to the position of the 

 anus, which is situated some way from the peristome upon the posterior coronal 

 process (PL XIX. figs. 8-10). It may be noted here that in Hall's diagnosis of 

 Eloeacrinus (Nucleocrinus) he says "the summit occupied by five or more plates," 

 but that he only figures five, of equal size, however, in his diagram of the structure 

 of the summit in Elasacrinus elegans z . These five plates of Stephanocrinus and 

 Elceacrinus have exactly the same relation to the peristome and ambulacra as the 

 oral plates of a Neocrinoid, and we do not see how their mutual homology can well 

 be disputed. 



Since the preceding paragraph was in type we have received the latest publication 

 of Messrs. Wachsmuth and Springer, according to whom the five plates which form 

 the ventral pyramid of Stephanocrinus are " calyx interradials," and cannot therefore 

 be homologous with the orals of a Neocrinoid 4 . In making this comparison Messrs. 

 Wachsmuth and Springer seem to have altogether overlooked the fact that Stephano- 

 crinus has well-developed calyx-interradials, namely, the deltoids (PL XIX. figs. 8-12). 

 This was explained by us in 1883 5 , and it is referred to again on page 34 of our 

 present work. 



The American authors regard the deltoid pieces of the Blastoidea, and by impli- 

 cation therefore those of Stephanocrinus, as homologous with the large calyx-inter- 

 radials of the Cyathocrinidae, a view in which we entirely concur, as we have explained 



1 'Revision of the Paheocrinoidea,' Part II. p. 16. 



- Archiv f. Naturgesch. 1850, Jahrg. xvi. Bd. i. Taf. v. fig. 3. 



Fifteenth Ann. Report, New York State Cabinet Nat. Hist. 1862, pp. 1 16, 153. 

 4 'Revision of the Palaeocrinoidea,' Part III. Proc. Acad. Nat. Sci. Philad. 1885, p. 268 (46). 

 6 Ann. & Mag. Nat. Hist. 18S3, vol. xi. p. 239. 



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