74 CATALOGUE OF THE BLASTOIDEA. 



on page 10. But in applying this name to the five plates which form the ventral 

 pyramid and cover the mouth of Stephanocrinus, and also of Ilaplocrinus and Allage- 

 cyiims, as they do in their latest publication, they seem to us to be going very much 

 too far. We regard the five summit-plates of all three genera as truly homologous 

 with the orals of the Pentacrinoid larva. They cover the mouth and the origin of 

 the ambulacra, just as the orals do in the Neocrinoid ; and this relation is not cha- 

 racteristic of the calyx-interradials in any Pelmatozoon whatever. It is only in the 

 Cyathocrinidse and in the Blastoids that these plates have any close relation to the 

 mouth at all. But they do not cover it and shut it off completely from the exterior, 

 as they summit-plates of Stephanocrinus and Allagecrinus do. For they form the 

 circumference of the peristome, from which the ambulacra pass outwards over their 

 apposed lateral edges (PL I. fig. 5 ; PL III. fig. 14 ; PL IV. figs. 1, 14 ; PL VII. fig. 14). 

 There is not a single Crinoid known in which plates that are universally recognized 

 as calyx-interradials cover in the actinal centre. The very name " calyx-interradials " 

 implies plates that are abactiual in their origin; while in Crinoids, Blastoids, and 

 Cystids alike we meet with types such as Allagecrinus, Stephanocrinus, and Glyptosplia- 

 rites, in which the mouth is covered by a pyramid of five closely fitting plates just as 

 in Neocrinoids {Hyocrinus), Urchins (PaUeostoma), and Holothurians {Psoitis). There 

 are therefore very strong reasons for regarding all these plates, which have the same 

 relations in five different classes of Echinoderms, as mutually homologous, i. e. as oral 

 plates 1 . One would like to know whether Messrs. Wachsmuth and Springer are 

 prepared to call the five plates which cover the mouth of Glyptosphasrites " calyx- 

 interradials," and if so, where the radials are on which they rest. 



The difference between Elceacrinus elegans or Stephanocrinus and E. Verneuili, as 

 described by Roemer, is very much the same as that between Culicocrinus and the 

 simplest form of Platijcrinus. Stephanocrinus, like Culicocrinus, has but five plates 

 in the vault ; while in Elceacrinus Verneuili there are at least seven, viz. an orocentral, 

 four proximals of equal size, and two smaller ones on the anal side. This is the 

 construction of the summit in the specimen figured by Roemer 2 , while in the best 

 preserved one which we have seen there is an additional plate in the proximal row 

 and one or two distal plates as well (PL XVIII. fig. 16). According to White 3 the 

 summit of Orophocrinus stelliformis consists of "five small plates, arranged like a 

 five-pointed star, with the points touching each of the upper ends of the interradial 

 plates," just as in Stephanocrinus and in Elceacrinus elegans (Fig. VI. A.). This 

 arrangement does not seem to be very constant, however, as it can scarcely be traced 

 either in Meek and Worthen's figure 4 or in the individual represented in our 

 PI. XV. fig. 12. 



1 See "ii this subject, Quart. Journ. Micr. 8ci. 1879, vol. \i\. pp. 191, L92. 



- Archiv f. Naturgesch. L851, lid. i. Taf. v. fig. 1 e. 



J Boston Journ. N'at. Hist. I Mi:}, vol. vii. no. I. p. 1-7. 



4 Report GeoL Survey Illinois, 1873, vol. v. pi. ix. fig. 5. 



