STRUCTURE OF THE CYSTIDE^. 36 



altogether of ambulacral and interambulacral plates, with the 

 exception of the small space in the back, consisting of the ocular 

 and genital plates. 



b. If a similar plane be extended through the lower edges of the 

 first primary radials of Pentacrinus caput-MeduscE, the body, as in the 

 sea-urchin, will lie above it ; but then it is covered at the sides by 

 the plates of the rays, and on its upper surface by the ventral plates, 

 all of which are antambulacral. 



c. In such Crinoids as the species of Poteriocrinus and Cyathocrinus, 

 which have two rows of plates below the radii, a plane through 

 the lower points of the primary radials would have about one-half 

 01 the body below it, and the other half above. Here we see a 

 tendency in the radiated skeleton to transfer its base from the dorsal 

 to the ventral side ; and it is interesting to perceive that in Crinoids 

 of this structure we first find species which indicate their approach 

 to the Cystideas by the exhibition of poriferous areas, which are at 

 least analagous to the pectinated rhombs of that group. These 

 exist in Porocrinus conicus (Billings) of the Trenton limestone, and 

 Caryocrinus ornatus (Say) of the Niagara group. 



d. The Cystideans with a definite number of plates, such as the 

 genera Ecldno-encrinites, Glyptocystites, Apiocystites and Prunocysiites, 

 are certainly more nearly related to the Crinoids than those with 

 the body covered by a large and indefinite number, as, for instance, 

 Echino-sjphcerites atirantium. The number five, so dominant in the 

 arrangement of the parts of the body in the Crinoids, is apparent in 

 these genera, although no trace of it can be seen in the sphaeronites. 

 Thus in E. angulosm (see plate iii. fig. lb) there are four basal plates, 

 but one of these is hexagonal, and can be divided so as to make two, 

 which wou'd be pentagonal like the other three. Above the basal 

 plates are three other series of five each. It may be granted that 

 the plates of the fourth series are the homologues of the five first 

 primary radials of Cyathocrinus planus (Miller), which form a circle 

 round the margin of the cup. When closely examined it will be 

 seen that in both the Crinoid and the Cystidean these plates are 

 notched in their upper margins by the ambulacral grooves ; and it 

 is therefore probable that they are strictly the homologues of each 

 other. 



If now in these two species we draw the plane as before, about 

 one-half the body of the Crinoid and nearly the whole of the Cysti- 

 dean will lie below it. The radiated skeleton in E. angulosus has 

 ascended from the base to the apex of the body, and it is there 



