1881.] NATURAL SCIENCES OF PHILADELPHIA. 205 



forming a part of a solid vault, are movable, and line the lateral 

 margins of the tentacle furrows. 



The proximal and central dome plates are altogether unrepre- 

 sented in recent Crinoids. This is best perceived by comparing 

 CoccocrnnuH as usually preserved, with Hyocrinus or other recent 

 genera in which the oral plates are developed. In both cases, 

 there is at the oral centre an opening at which the grooves con- 

 vei-ge, surrounded by the oral plates ; but, while in Hyocrinus and. 

 all recent genei-a this opening is unobstructed b}" solid parts, in 

 Coccocrinus, Gyathocrinus^ and the Palaeocrinoids geuerall}', it is 

 covered by the apical dome plates. The central piece generally 

 occupies the median portion of the vault, and always indicates 

 the centre of the oral system. 



We have already noted narrow grooves upon the inner surface 

 of the vault, which meet on the central piece, and follow the 

 median line of the radial depressions and galleries to the arm 

 openings. Only three main grooves meet at the centre, those of the 

 two lateral rays are uniting before reaching that point (PI. XVIII, 

 fig. 1). The grooves are best observed in natural casts of the 

 interior, in which they appear on the surface in the form of narrow 

 bands or ridges (PI. XIX, figs. 5 and 9). The position of the 

 grooves indicates that they may have contained axial cords in 

 connection with a nervous system located beneath the central 

 plate. The location of the nervous system within the regions of 

 the ambulacral centre is in analogy to the structure of other Echi- 

 noderms, except the Comatulidffi, in which, according to P. 

 Herbert Carpenter, the principal nervous systems are located at 

 the apical side, and in connection with the quinquelocular organ 

 which occupies the cavit}- of the centrodorsal plate. ^ 



The interpalmar fields are composed of a soft skin, but although 

 this is more or less incrusted with limestone particles, which 

 sometimes almost look like vault pieces, they have no affinities 

 with the plates of the vault. The plates of all recent Crinoids 

 are perforated with numerous pores for the introduction of water 

 into the body, a function which could not well be pei'formed by 

 the interradial pieces, but much less by the solid undivided oral 

 plates of Gyathocrinus and Goccocrinus. In the Cyathocrinidse, 

 these functions may have been performed by the ventral sac 



1 On some points in the anatomy of Pentacrinus and lihizocrinus. Jour. 

 Anat. and Phys., vol. xii p. 35. 



