1885.] NATURAL SCIENCES OF PHILADELPHIA. 275 



summit, as it covers the mouth, and lodges underneath the 

 annular vessel, which is the origin and centre of the whole 

 ambnlacral system. As such it has not only the position but 

 performs the functions of the closed oral pyramid in the Penta- 

 crinoid larva. Wh} r , therefore, should the proximals be the 

 orals, and the central piece represent something else that is 

 totally unknown in Crinoid ontogeny, and among Echinoderms 

 generally ? The proximals, as a rule, surround the peristome, 

 but do not cover it. The tentacular vestibule is closed by the 

 central piece. This is well shown in our specimen of Batocrinus 

 Christyi (PL 5, fig. 6), in which the perisomic plates extend up 

 to the central piece. Other specimens (PI. 4, fig. 4, and PI. 8, 

 figs. 1, 2, 5) show that the radiations pass out from beneath the 

 central piece, and not from beneath the proximals. If there 

 had been such a thing as an " orocentral," it is difficult to 

 understand how this plate could have entered the " oral ring," 

 unless it was developed in the early larva, as the proximals 

 remain permanently closed. The Palseocrinoids, as a rule, 

 have a central piece, but they do not all have proximals, 

 and it is very significant that the proximals are absent in the 

 earliest Silurian genera, and are most conspicuous in the later 

 and higher types. Eeterocrinus juvenis is evidently in the same 

 morphological condition as Haplocrinus. The ring of plates, 

 which Carpenter no longer considers orals in Cyathocrinus, 

 encloses a central piece without proximals, and in all probability 

 the same is the case in Hybocystites and the Hybocrinidae gener- 

 ally. The Reteocrinidae possess on\y a small central piece, but 

 have no proximals. Are the orals here resorbed, and also the 

 interradials ? That would, indeed, suggest a very peculiar condi- 

 tion for a Lower Silurian genus. 



The basals, as pointed out by Carpenter, are the most important 

 plates in the calyx. They lodge within their cavity, bounded by 

 the radials, the" chambered organ, which is the centre of the 

 nervous and vascular system, and from the basals the axial canals 

 pass out to the radials and arms. In the summit, the central 

 plate occupies, in relation to the radials, the same position as the 

 basals. It is the only summit plate that is represented in every 

 Palaaocrinoid, and it lodges underneath the most important 

 organs of the oral system. In view of these facts, and admitting 

 that the orals are the homologues of the basals, there can scarcely 



