1885.] NATURAL SCIENCES OF PHILADELPHIA. 267 



finding its application somewhat cumbersome, as the word 

 " apical " is used in a different sense, we have abandoned it. The 

 summit plates are represented in the PaUvoerinoidea by the central 

 piece, the six or more so-called proximate, and the radial dome 

 plates; in the Neocrinoidea, by the oral plates alone. 



The orals constitute important elements in the ontogeny of 

 recent Crinoids. They appear at first in the form of a closed 

 pyramid, composed of five triangular plates. 



According to Dr. P. H. Carpenter (Chall. Rep., p. 71), " their 

 rudiments appear in the free-swimming larva simultaneously with 

 those of the basals, which are developed spirally round the right 

 peritoneal tube ; while the orals appear in a similar spiral around 

 the left one. The skeleton is at first limited entirely to these 

 two rings of plates, the edges of which meet around the equator 

 of the growing cup, though they ultimately become separated by 

 the appearance of the radials between them. At the base of the 

 closed pjn-amid formed by the oral plates is the upper portion of 

 the larval body, in the centre of which the opening of the mouth 

 is formed. . . . At a certain period of development, the five 

 valves of this oral pyramid gradually separate so as to open the 

 mouth to the exterior, and allow the protrusion of the tentacles, 

 while the floor of the original tentacular vestibule, with the 

 mouth in its centre, becomes the peristome of the growing 

 Crinoid." Afterwards the orals become " completely separated 

 from the basals and radials by the equatorial perisome and are 

 relatively carried inwards, while the second radials project some- 

 what outwards. . . . The orals are thus left as a circlet of five 

 separate plates protecting the peristome in the centre of the 

 upper surface of the disk." In all Pentacrinidse and also in the 

 Comatulre, with the single exception of Thaumatocrinus, the orals 

 eventually undergo a process of resorption, while in Ehizocrinus, 

 HyocrinuS) HoIojjus and Thaumatocrinus, the}' persist through 

 life. 



Nothing is known of the orals in Mesozoic Crinoids. 



That the oi'als, which assume such an early prominence in the 

 ontogeii3 T of the later Crinoids, should be unrepresented in 

 paheozoic ones, seems scarcely possible. This has been conceded 

 by various writers, but there is, as yet, much difference of opinion 

 as to the plates which represent them. 



The first writer who referred to oral plates in palaeozoic 



