148 
THE GEOLOGIST. 
star of five regular rays, or a little cup with the edges cut oflf and 
angular. Those plates alternate with five sub-radial plates of hexagonal 
form, but of which two of the lateral sides are sometimes so slightly 
developed that there appear to be but four, and in this case they affect 
the form of a lozenge. The two other plates are very much larger; 
the one, sub-quadrangular, is set directly as the support to the first 
radial plate, and the other, of an irregular sub-pentagonal shape, sup- 
ports one of the anal plates, which, to the number of five, are disposed 
so as to occupy a limited space below, at the base, and on the two 
sides by the radial plates, which in the two adjacent rays precede the 
origin of the arms. 
The first radial plates are two in number, and are very much alike 
in the five rays. Each ray bifurcates in its turn, and each of the arms 
to which it gives rise is composed of five plates, of nearly equal length. 
The axillary plate is surmounted by two arms, each formed by the 
union of about two hundred articulations, alternating and attached 
laterally to each other. The combination of all these arms produces a 
sort of tube or cylinder, of an elongated form, terminated by a 
circle of fifteen plates, attached laterally together, serving as the 
exterior limit of the dome, which is slightly elevated, and com- 
posed of a great number of little pentagonal or hexagonal plates, 
of which neither the form nor disposition is at all regular. I have 
not been able to observe any trace of a horn or proboscis. The 
stem is of a cylindrical form, and composed of articulations 
of a diameter alternately larger and smaller, which make it appear 
ringed. 
Affinities and Differences. — If one had under his eyes only the inferior 
part of the head or bulb of Hydreionocrinus, it would be impossible to 
distinguish it from that of Poteriocrinus ; the disposition and the number 
of the different basal, sub-radial, radial, and anal plates being exactly 
the same in the one as in the other ; but, whilst in the Poteriocrinus the 
arms are generally rather long and entirely free, in Hydreiocrinus they 
are soldered together in all their parts in the form of a cylindrical tube 
surmounted by a dome, of which there exists not a trace in the former, 
and which seems to be wanting in the species of the proposed new 
genus. 
The perfect resemblance of the inferior parts of the bulbs of the two 
genera now named has been the cause of certain species, of which the 
