148 



THE GEOLOGIST. 



star of five regular rays, or a little cup with the edges cut off and 

 angular. These 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 Differ ences.^li 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 



