28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



2. Orals a Spherical triangle. Bathymetric Thermal 



range range 



Pentacrinitida: (except Calometri- 



dse) 0-2900 28.7-80.0 



Holopodidre 5-120 71.0 



Plicatocrinida; (Ptilocrinus) 266-2485 35.3 



In the earlier crinoids (except the Flexibilia) the orals were rela- 

 tively thick plates lying- in the teg-men, of which they formed part 

 of the plated surface, and hence they acquired more or less the form 

 of spherical triangles of very appreciable thickness. The disintegra- 

 tion of the orals, following that of the pavement of the disk, took 

 place from the periphery of the oral circlet, gradually working toward 

 the center. As the orals became thinner and thinner dorso-ventrally 

 it naturally resulted that their edges bordering the ambulacral 

 grooves, which were the last portions to be affected, became promi- 

 nent, standing up above the general surface as thin blade-like borders 

 parallel to the dorsoventral axis of the disk of gradually increasing- 

 height, the orals eventually disappearing- not as horizontal plates 

 lying in the tegmen but as five trough-like structures surrounding 

 the mouth with their angles in apposition, and with their longest 

 dimension, representing the long dimension of the trough, parallel 

 to the dorsoventral axis. Orals of this type are characteristic of the 

 pentacrinoid young of the macrophreate comatulids. 



But while the reduction and disappearance of the orals after their 

 complete formation as skeletal structures characteristic of the adults 

 took this course, reduction of the orals gradually was shoved further 

 and further back into the ontogeny of the later types so that it set 

 in before the orals commenced to thicken. Cessation of development 

 of the orals at this stage leaves them in the form of very thin plates 

 lying in, and conforming in contour to, the inner angles of the 

 interambulacral areas. 



Thus the presence of thin orals lying in and conforming in contour 

 to the inner angles of the interambulacral areas is an indication of 

 an advanced stage of suppression of those plates, which has been 

 shoved far forward into the ontogeny. So far as we know the orals 

 of the stalked young of the oligophreate comatulids never develop 

 further than this point. 



