External Structure of Paleozoic Crinoids. 455 
nus is allied to Melocrinus, and has undoubtedly a similar 
summit-structure. The same may be said of Schizocrinus 
and Dimerocrinus, which are not at all related to Cyatho- 
crinus. 
The genus Cyathocrinus was originally described by Prof. 
Phillips and Mr. Austin as having a separate mouth and vent, 
which was considered by these authors and others to be its 
chief distinction from Poteriocrinus. Accordingly, all species 
with a proboscis or solid dome, though otherwise agreeing 
with Cyathocrinus, were referred to Poteriocrinus or some 
allied genus. Meek and Worthen, however, proved (in the 
Geological Report of Ilinois, vol. v. p. 325) that in perfect 
specimens the central opening is closed. The covering of 
Cyathocrinus is exceedingly interesting, and throws light 
upon the summit-structure of many genera. I shall herein 
refer frequently to Meek and Worthen’s excellent figures, 
vol. v. pl. ix. figs. 13, 14. 
Looking only at fig. 14, one would at first naturally suppose 
there must have been, during the life of the animal, two dis- 
tinct openings in the vault. But on examining it more criti- 
cally, and comparing it with fig. 13, it will be found that 
fig. 14 represents simply the consolidating apparatus as 
figured by Roemer and Schultze in Cupressocrinus, placed 
here exactly as in that genus, and consisting of five large 
pieces, alternating with the upper edges of the first radial 
plates. The plate of the anal side is larger than the others, 
and forms the base of the inner side of the proboscis. The 
five pieces, which connect with each other laterally, extend 
inward for some distance, but not so far as to meet in the 
centre, where there is a semicircular or heart-shaped opening. 
Along the sutures between the five plates a comparatively 
large furrow from each arm-base extends inward, and leads 
to the central opening. Examining now fig. 13 we find the 
general aspect of the ventral disk entirely changed. The 
lateral opening has been transformed into the base of a pro- 
boscis ; and the consolidating plates are partly covered, leaving 
but a small uncovered space, in the form of a delta, in the 
interradial areas. The central opening is vaulted over by a 
number of various-sized pieces, the largest one occupying the 
side towards the proboscis. The shallow groove between the 
sutures of the consolidating plates is arched by a double series 
of alternating plates, forming underneath a passage for the 
ambulacral canal and food-groove. ‘The vault, thus closely 
resembling that of Synbathocrinus, was in all probability ar- 
ranged on a similar principle in Cupressocrinus. The same 
‘plan, with slight modifications, prevailed in Potertocrinus, 
