588 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



But in the following only the pinnules beyond the ninth-twelfth brachial and that 

 on the second brachial are present : 



Comactinia merndionalis. Antedon bifida. 



PtUometra muUeri. Antedon raediterranea. 



Antedon adriatica. 



Side and covering platen. — From their first appearance the side plates 

 and covering plates of the pentacrinoids resemble very closely those of the adults. 



In Promachocrhius kerguelensis side plates and covering plates accompany 

 the IBr, and IBr2, as well as the brachials and pinnulars. In Antedon petasus, and 

 probably also in many other species, they accompany the IBr„. 



In the pentacrinoids of the following species the side plates and covering 

 plates (usually in the form of side plates with incompletely differentiated covering 

 plates) are large and conspicuous: 



Crotalometra porrecta. Eeliometra glacialis. 



Glyptometra tvherosa. Promachocrinus kerguelensis. 



They are moderately large in Antedon petasus; they are very rudimentary and 

 delicate, and usually very narrow, consisting of a few rods arranged in a loose 

 meshwork in 



Comactinia meridionalis. Antedon bifida. 



Comissia littoralis. Antedon mediterranea. 



Antedon adinatica. 

 they are reduced to slender rods which may be branched in 



Hathroinetra prolirra. Hathrometra sarsii. 



they do not occur in Anthometra adriani; and they have not been noticed in 

 Ptilometra ntiilleri. 



Orals. — While the outline of the orals is always triangular, so that when they 

 are closed together they almost or quite conceal the mouth, their surface shows con- 

 siderable diversity of configuration. 



In the very young the orals are always spherical triangles, more or less strongly 

 curved inward in the outer portion. 



Subsequent addition to their free lateral borders takes place so rapidly that 

 if the original plan of a spherical triangle were maintained they would soon come 

 to overlap; therefore the edges as they form turn outward more or less abruptly, 

 according to the difference in the increase of the surface of the orals as a whole as 

 compared with the increase in the surface of the disk. 



If the growth of the orals is comparatively slow the eversion of the edges 

 makes only a small angle with the surface of the original spherical triangle, as in 

 Comactinia meridionalis; but if it is relatively rapid the new calcareous deposit is 

 abruptly everted, so that it is laid down in planes which include the dorsoventral 

 axis of the animal, as in Hathrometra prolixa. 



