Till; SUMMIT-PLATES. - I 



aperture (PI. XIX. fig. 9). In Pentremites conoideua the other four spiracles arc 

 perhaps also closed by plates ; and except in the larger size and abundance of the 



plates it is no great advance from this condition to that which we have seen in 

 Pentremites aulcotua (PI. I. fig. 8), but we await further information. 

 For some little time after the discovery of the summit-plates of the Mastoids, no 



special attention was paid to their morphological importance. It was natural enough to 

 compare them, as some authors did, to the vault of the Actinocrinida 1 ; hut no detailed 

 comparisons were made until, in 1877, Wachsmuth 1 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, with five or, more frequently, six others disposed 

 interradially round it. Four of these six are of tolerably equal size, while the other 

 two are smaller and situated in the anal interradius, being separated from each other 

 by the anal plate or by the proboscis. Wachsmuth has described these seven pieces as 

 the ''apical dome-plates," and finds that they are represented in the vault of most. 

 if not all, PaUeocrinoids. They are relatively larger in young individuals, and in 

 older specimens are sometimes altogether indistinguishable from the other plate- of 

 the vault, which are developed outside them. But Mr. Wachsmuth's researches 

 leave little doubt that they are present, at any rate in early life, in all the more 

 typical forms of Actinocrinidae, Platycrinidae, and their allies, which he L r r<>up> 

 together provisionally under the name of Spbteroidocrinidie. 



He further points out that these apical dome-plates "surmount the vault ol 

 Symbathocrinus and Cyathocrinus, cover the central opening of the Blastoids, and can 

 be traced in many of the Cystideans." He also suggested that "the centre piece 

 corresponds evidently with the basals of the dorsal side, the surrounding plates to the 

 subradials (the two smaller plates, separated by the anus, forming together one large 

 one)." In the more rational nomenclature which is now in almost general use for the 

 calyx of a Crinoid, the centre piece would be compared to the under-basals, and the 

 proximal dome-plates to the basals. In 1879 Dr. P. H. Carpenter 2 suggested that. 

 as the under-basals are somewhat variable elements in the calyx of a Crinoid, it would 

 be better to substitute for them the central plate of the abactinal system of an 

 Ecbinoderm, or '• dorsocentral," as the bomologue of the central dome-plate, which 

 he subsequently proposed to call the " orocentral " 3 . We understand that Mr. 

 Wachsmuth has now abandoned his original view that the proximal dome-plates are 

 homologous with the basals of the abactinal side. But it is has always appeared to us 

 that this was a peculiarly fortunate suggestion, and one which threw a flood of light 

 upon the structure of the summit in all the Crinoidea. For it has been shown by 



1 Amor. Journ. Sci. 1877, vol. xiv. pp. 186- L89. 



2 "On the Apical and Ural Systems of the Echinoderaata," Part LL, Quart Journ. Micr. Sci. - 

 vol. xix. pp. 181, 182. 



3 Zool. C'hall. Exp. part xxxii. 1884, p. 169. 



