230 PROCEEDINGS OP THE ACADEMY OP [1881. 



calyx, and the plates of the different rays meet laterally in the 

 body. The case is simihir to that of Slrotocrinus ; the arms, as 

 in that genus, are crowded together, naturally producing at first 

 a lack of mobility in the proximal parts of the rays, until eventu- 

 ally the sides became attached. In P. crassus and P. Chester- 

 ensis Meek and Worthen, in whicli the lower arm portions are 

 comparativel}"^ narrower, the connection between the ra^^s is not 

 perfect, especially at tlie posterior side ; while in P. depreasus 

 Lyon, and in all of Wetherby's species, the connection between 

 the upper radial is uninterrupted, and the arms are given off 

 directly from the body, and not from brachial appendages or free 

 rays. In Meek and Worthen's species, in which the first radial 

 plate extends to the top of the calyx, it is apparent that the first 

 plate above, thougli pushed into the dome, is the analogue of the 

 regular interradial plate of the group, but in P. depressus^ in 

 which that plate rests above the tertiary radials, it is probable that 

 the interradial plate proper was pushed inwards, and either 

 became obsolete, or is perhaps visible only at the inner side of 

 the test. 



The construction of the dome has been already so fully dis- 

 cussed that a few general remarks here will suffice. The vault 

 resembles fundamentally that of the Actinocrinidse and Rhodo- 

 crinidoe. The plates are comparatively large, and the apical dome 

 plates very conspicuous. The radial regions are each composed 

 of two rows of plates alternately arranged, which commence 

 either close to the centre plate, or near the edge of the disk, and 

 branch toward the free rays, following their direction, and paving 

 their ventral surface. The interradial regions of the dome are 

 comparativel}'^ large, composed of one, two or more plates. 

 Anus in form of a small tube, <w simple vault opening. Column 

 round or elliptic, never pentagonal ; perforation very small. 



For greater convenience we have divided the Platycrinida^ into 

 two sections : 



A. Platycrinites, including genera with a marked pentahedral sym- 



metry, and without special anal plates in the calyx. 



B. Hexackinites, with a decided bilateral symmetry, and a large anal 



plate enclosed within the calyx. 



The presence of a special anal plate, upon which this division 

 has been based, is somewhat remarkable, on account of the great 



