372 PROCEEDINGS OF THE ACADEMY OF [1888. 



the palaeozoic crinoids, and proves that, while the groups themselves 

 are entirely correct, our error consisted simply in a wrong under- 

 standing of the fiimily, which led us to assign it to a group to which 

 it does not belong. 



Let us now proceed to ascertain to what group Crotalocrinus 

 should be assigned. 



We established the suborder Articulata to include the grouj^ 

 defined by us under the family name Ichthyocrinidae with the addi- 

 tion of Crotalocrinns and Enallocrwus, (Revision III, p. 140). It is 

 clear from what we now know of their structure, that the two latter 

 cannot remain among the Articulata as that suborder has been 

 defined by us. * 



There is no doubt that Crotalocrinus possesses some characters 

 belonging to each of the three groups which we introduced in the 

 third part of the Revision. It resembles the Articulata in the peculiar 

 articulation of the arms. In the arrangement of some of its calvx 

 plates it bears a very close relation to the Inadunata, especially 

 Cyathocrinus, to which genus J. S. Miller referred it. Like that it 

 has three rings of plates, the upper one including a single anal plate. 

 A casual glance at the structures succeeding these would lead one to 

 think them somewhat similar to the unconnected rays of the Inadu- 

 nata, but a more careful study will show that they are constructed upon 

 the same princij^le as the same parts in some groups of the Camarata. 

 They are actually neither true radials nor free arm plates, but, as in 

 the Platycrinidae, represent a transition between them. As in Platy- 

 erinus there are but two primary radials, the upper one a small 

 triangular axillary, to both sides of which the secondary radials are 

 attached, resting both against its sloping sides and upon the first 

 radial. They are overlaid by the tertiary radials, of which the lower 

 ones often, in a similar way, rest upon the secondary radials and the 

 first primary. 



All these plates, in a most peculiar and unique manner, are solidly 

 fitted together with each other and the first primary radial, and have 

 at their ventral face a wide, deep, diverging groove, arched by strong, 

 rigid covering plates, with a large tubular cavity underneath, which 

 in reality forms a part of the cavity of the calyx, like in the free 

 radial appendages in some forms of the Platycrinidae and Actino- 



* The actual discovery by us of the disk of Taxocriniis witli an external 

 mouth, which we have elsewhere described, has settled all debate as to the summit 

 structure of the Ichthyocrinidae. 



