980 MB. r. A. BATHER ON UINTACKINUS. [DeC. 17, 



the dorsal cup consist of (i.) prirnibrachs, IBr ; (ii.) seciindibrachs 

 or distichals, UBi- ; (iii.) fixed piaaules. By " fixed " one means 

 that the ossicles in each longitudinal series are attached not merely 

 to one another by their upper and lower margins, but also to the 

 ossicles o£ adjacent series by their lateral margins, thus forming 

 the cup. The precise limits of fixation are not very easy to 

 determine in the fossil state : for instance, the distal extremities 

 of the earlier pinnules were undoubtedly free, although their 

 proximal portions were firmly united laterally to the distichals or 

 to other pinnules ; but the exact point at which lateral union 

 ceased cannot be determined, since, even above the limits of strict 

 lateral sutures, the pinnules may have been laterally united by a 

 membrane. The same uncertainty prevents us from saying at 

 what level the arms became free; but we may say somewhere about 

 the 8th and 9th distichal, IIBrg. 



The primibrachs {"■ radials of the first order " in part, or 

 " costals," of some writers) are two in number (see figs. 4 and 5). 

 IBrj is hexagonal, resting by its lower edge on the feebly concave 

 upper margin of the radial, supporting IBr^ on its upper margin, 

 and abutting laterally on two interbrachials on either side. It is 

 slightly less wide than the radial, but about the same height. 

 IBr, is axillary (lAx) and pentagonal, having on rare occasions an 

 irregularity in the lateral margin, owing to the abutment thereon 

 of more than one inter brachial. 



The fixed secundibrachs (or fixed distichals) may, as aforesaid, 

 be reckoned at about 8. They are fixed by means of interbrachials, 

 fixed pinnules, and interdistichals. Owing to the origin from 

 some of them of the fixed piunules, they have a slightly irregular, 

 zigzag arrangement, and those that bear pinnules have somewhat 

 the aspect of axillaries. The law of their pinnulation, in both 

 right and left branches of each arm, may be stated thus : IIBrj, 

 none ; IIBr^, outer ; IIBrg, none ; IIBr^, inner ; IIBr., outer ; 

 IIBr^, none ; IIBr^, inner ; IIBr^, outer ; IIBrg, none. This, at 

 least, is the arrangement I have found in eight specimens out of 

 ten, e.g. specimens y, d, e,f, g, I (figs. 4, 5, 10). One of the ten 

 specimens, viz. h (fig. 6), varies thus : IIBr^., inner ; IIBr^, none ; 

 IIBr,, outer ; IIBr,,, inner. The remaining one, viz. p (fig. 7), 

 varies thus : IIBr^, none ; IIBr^, outer ; IIBr^, inner ; IIBr^, none ; 

 IIBr^, outer. In both these cases the regular alternation of pin- 

 nules remains. It appears that all the rays of any individual vary, 

 if they vary, according to the same plan. 



The above results were gained from an examination of actual 

 specimens in the British Museum : it is instructive to compare 

 them with the published figures. Grinnell's (2) figures 1 and 2 

 (my fig. 1) agree with the law so far as the portions in question 

 are preserved. Both of Meek's (3) figures (my fig. 2) agree with 

 it in every particular. Prof. Clark's (8) " Diagram showing the 

 structural arrangement of the plates in the test " agrees with it. 

 Clark's pi. i. fig. la (PL LVI.) shows the following arrangement 

 in two branches : — IIBr,, outer ; IIBr^, none ; IIBr,, outer. Not 



