STRUCTURE OF CALYX AND ARMS. 



THE ARMS AND PINNULES. 



The first observation of these specimens made clear the fact that the arms 

 are pinnuliferous, — a fact which has been overlooked in the discussions here- 

 tofore ; although, as already stated, Billings, in his description of C. regius, 

 Dec. IV., p. 56, mentions the presence of " tentaculated, free rays." 



The structure of the arms is perfectly shown by specimen C (PI. I., 

 Fig. 3 a). They are direct continuations of ridges which follow the 

 brachial series in the calyx, beginning at about the first bifurcation with 

 faint linear elevations, and gradually enlarging until they pass into the free 

 arms. These are quite angular on the back, simple, uniserial, composed 

 of short, wedge-form plates, each of which gives off a pinnule from the 

 longer end. The pinnules are slender, and lie close together (PL I., Fig. 10). 

 At somewhat irregular distances below where the arms become free, at the 

 outer margins of the rays, there are given off subordinate branches, which 

 originate on a bifurcating plate and lie closely alongside the outer arms of 

 the ray, the plates of the two abutting and interlocking. These subordinate 

 branches are the proximal portions of fixed pinnules, which are incorpo- 

 rated in the calyx walls, and apparently become free between the arm 

 bases, though they are concealed from view by the regular pinnules. In 

 other interbrachial spaces higher up, from the last IIIBr or IVBr, and 

 mostly on the outside of the dighotom, similar pinnules are given off, but 

 not with entire regularity (PI. I., Fig. 8). 



All these are closely joined together and to the adjacent arms, and 

 incorporated into the calyx walls, until they reach the upper margin and 

 become free. The arms are quite clearly shown in Billings's type specimen 

 (PL I., Figs. 1, 2), where the bases for a distance of several brachials are in 

 some places well preserved, having the pinnules attached, — though neither 

 of them is as well shown as in specimen C. 



