556 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



large percentage of the recently detached young have only the 10 radial, or the 

 10 radial and 2 (posterior) interradial arms. Since all of the fully grown speci- 

 mens recorded have 20 arms, it would appear that these 10 or 12 armed young 

 never reach full size. 



Brachials. — The brachials do not differ appreciably from those in the young 

 of other species. 



Covering plates. — Each brachial as soon as it appears is accompanied by a 

 pair of large, though thin, covering plates, composed of a large and irregular 

 main portion (side plate) and a smaller incompletely differentiated distal portion 

 (covering plate proper). 



These covering plates not only accompany all the brachials but are also found 

 on either side of the IBr, (axillaries) and even sometimes on either side of the 

 IBri, where, however, they are much reduced in size. 



Pinnules. — The first pinnules appear, apparently about the time of the appear- 

 ance of the first cirri, on the twelfth or thirteenth brachials, and are soon followed 

 by the pinnule on the second brachial. All of the pinnules from their first incep- 

 tion resemble more or less closely the perfected form. 



On the distal pinnules the third-fifth segments bear seven very large and 

 conspicuous covering plates, which in height considerably exceed the lateral 

 diameter of the plates bearing them. 



Orals. — The orals of Promacliocrinus are probably at first, as is usually the 

 case, spherical triangles. But sometime before the appearance of the radial 

 structures the original plan of the orals is suddenly changed, and the extension of 

 their free lateral borders takes place at an abrupt angle to the surface of the 

 plate as first laid down in such a way that further growth is in planes which 

 include the dorsoventral axis of the animal. Each oral thus consists of a spherical 

 triangle with abruptly upturned edges gradually tapering to a point at the 

 proximal lateral angles, the upturned borders of the two outer sides being sepa- 

 rated by a deep and narrow notch, the point of which rests upon the distal apex 

 of the original spherical triangle. 



At about the time of the first formation of the radial structures this plan 

 of growth changes. The notch between the everted lateral extensions becomes 

 filled with a new calcareous reticulation, which, growing more rapidly than the 

 lateral extensions, gradually comes to form a produced narrowly rounded tip to 

 the oral, this new growth as a whole eventually becoming rhombic in shape, either 

 flat or gently concave (as viewed from the exterior), the rounded anterior angle 

 of the figure forming the distal angle of the oral, while the proximal sharp angle, 

 which rests upon the distal apex of the original primitive oral, represents the 

 filling in of the original notch, now greatly increased in size through the con- 

 tinued extension of tlie lateral borders. 



As the same time these lateral borders, originally formed in planes including 

 the dorsoventral axis of the animal, have become curved inward in order always 

 to maintain the connection with the growing, more or less flattened, distal rhombic 

 area, so that now in lateral view their edges are seen to make a broad curve from 

 the proximal angle of the basal to the outer angle of the rhombic area. 



