MORPHOLOGICAL PART. 83 



increases with the size of the specimens, and that with the increase of the 

 latter additional brachials are incorporated into the calyx. We have found 

 a number of young specimens in which the arms are free from the first 

 axillary, but, as yet, no AcUnocnmis or Bhodocrinus in which there is not at 

 least one interbrachial. 



. We find among the Camerata a large number of transition forms in 

 which the costals do not form a part of the dorsal cup, and the first inter- 

 radials, to a large extent, are interambulacral in position. Such is the case in 

 the Platycrinidae, Hexacrinidse, Acrocrinidse, and Crotalocrinidae. In Platy- 

 crinus the distal ends of the radials are provided with a horse-shoe-shaped 

 excavation, which closely resembles the facets of the Inadunate Cyatho- 

 crinid^. This facet, which in some species extends down to two-thirds the 

 length of the plate, rarely takes up more than half its width. It is occupied 

 by the costals, or, when these are very small, in part also by the first dis- 

 tichals. The costals are narrow, and resemble in form and general appear- 

 ance the free brachials of Inadunate Crinoids. They are, however, not free 

 in the sense of these, but the proximal ends of one or both plates are sutur- 

 ally connected with the plates of the tegmen, so as to be immovable, and 

 only their upper ends are free from the calyx. 



It has been generally supposed that there was in Platycrimis and Dicho- 

 criniis an articulation or loose union between the radials and costals. This 

 seemed to be confirmed by the presence of the facet, and by the fact that in 

 some species this is provided with a sort of imperforate transverse ridge, or 

 angularity. We have no doubt that these costals w^ere movable at some 

 period of life, probably in the larva before the perisome made its appearance, 

 and the orals still rested upon the beveled edges at the outer ends of the 

 radials. But it is equally certain that the plates were immovable in the 

 adult, owing to the rigidity of the interradial and covering pieces, with which 

 they are suturally connected. The systematic position of the Platycrinidse is 

 intermediate between the Inadunata and the typical Camerata. They repre- 

 sent phylogenetically an early stage of Actinocnmis, Batocrmiis, and their 

 congeners; but are nevertheless true Camerate Crinoids, for their lower 

 brachials are fixed, retaining, however, permanently to a large degree the 

 characteristics of free arm plates. In Cidicoerinus and Pterotocriniis, the one 

 a Platycrinoid, the other a Hexacrinoid, the Camerate type is somewhat 

 more advanced, as their costals practically enter the dorsal cup ; and there 

 are a few species oi Flatycrinm in the same condition (Plate LXXI., Fig. 16). 



