MORPHOLOGICAL PART. 95 
It remains to consider the views of Neumayr. As already stated, he 
agreed with Carpenter that the Scheitelplatten of Haplocrinus, and the six proxi- 
mals of the Camerata, represent the orals. He also believed in the presence 
of a central plate within the oral ring in both groups. But he differed 
both from Carpenter and us as to the plates representing the orals in the 
Cyathocrinide. 
The structure of the ventral disk of Cyathocrinus exhibits considerable 
variability, and a comparison of the various plates among the different 
species is by no means an easy matter. As a rule, there are four large 
interradial plates located ventrally, resting upon the inflected upper edges 
or limbs of the radials, and at the posterior side two narrow longitudinal 
strips, which enclose a large, perforated madreporic plate lying in front of 
the ventral sac, and whose lower (outer) edge is in contact with the sac. 
The plates are laterally united by suture, and leave five well defined grooves 
which are occupied by the ambulacra. Within these plates, towards the oral 
centre and covering it, there is a variable number of other large plates, often 
of the most irregular arrangement, varying in form even in the same species, 
and in some cases exhibiting the asymmetry of the orals in the Camerata. 
The difficulty of ascertaming the morphological relations of these plates in 
the different forms is increased by the fact that the surface of the outer 
plates — those nearest the radials — is covered by numerous minute peri- 
somic pieces, interposed between the ambulacra. The ambulacral plates 
consist of side plates and covering pieces. 
Neumayr * speaks of only one ring of plates, resting against the radials 
and surrounding the mouth, and he assumes that the disk of Cyathocrinus 
is morphologically in the condition of Haplocrinus, except that the ambu- 
lacra in the latter are subtegminal, but tegminal in the other. We have 
illustrated on Plate III the ventral structure of Cyathocrinus by a series 
of specimens of different geological ages, and in various stages of preser- 
vation, which show that the disk is composed of two sets of plates, the 
one within the other, and that in cases where but one ring is visible the 
plates of the second are covered by other structures, or have been resorbed, 
or are not preserved in the specimen. It is evident that Neumayr has in 
some instances confounded the plates of one ring with those of the other. 
Examining first the two specimens of C. Giles? (Figs. 1 a, 4), there appears 
to be but one ring of plates, and these rest against the radials. In Fig. la 
* Die Stamme des Thierreiches, pp. 449-452. 
