ACTINOCRIJflDiE. 553 



Calyx distinctly lobed at the arm regions, the higher orders of brachials 

 forming, together with the ambulacral plates overlying them, five more or 

 less prominent calicular extensions, or brachial lobes, from the two divisions 

 of which the free arms are given off by alternate bifurcation from every 

 second or third plate. The plates of the dorsal cup are ornamented by radi- 

 ating ridges, which often meet in a node. Basals three, rather large, forming 

 a well defined cup. Radials larger than any of the succeeding plates. Cos- 

 tals two, the first hexangular — exceptionally quadrangular. Distichals all 

 axillary ; supporting at their outer side an arm, at the inner from two to three 

 palmars. The succeeding bifurcations take place in a similar manner ; each 

 successive order of brachials consists of two or three plates, of which each 

 axillary supports an arm at one side, and two or three post-palmars — accord- 

 ing to species — at the other, and so on to the last bifurcation in the calyx, 

 which gives origin to two arms. When there are only four arms to the ray, 

 the last bifurcation is on the distichals, and there is of course no alternation. 

 The arms are given off alternately from opposite sides in almost the same 

 way as the pinnules from an arm with a succession of syzygies; but in 

 Actinocnnus the sides of the proximal arm plates are suturally united with 

 the plates which support the succeeding order of brachials. Arms biserial, 

 and frequently branching once or twice after becoming free; the pinnules 

 long, and their proximal joints armed with a small hook, projecting from 

 the middle of the plates. The interbrachial spaces on approaching the 

 arm regions are widely and deeply depressed; they are occupied by numer- 

 ous plates, which meet with the interambulacral pieces above, and in some 

 species take part in the formation of the brachial extensions. The anal side 

 is the widest, containing one large plate in line with the radials, which is 

 followed by two interbrachials ; the anus is located at the distal end of a 

 strong tube, which rises abruptly from the summit of the tegmen. Orals 

 rather small and excentric. The ambulacral plates either consist of two rows 

 of large covering pieces, which pass out from between the orals and follow 

 the branching of the food grooves, or are represented by large single plates 

 of a first and second order, succeeded by small covering pieces, which meet 

 with those of the free arms. Column round, the axial canal small and 

 pentangular. 



Distribution. — Restricted to the lower part of the Subearboniferous or 

 Mountain limestone. In America it makes its appearance in the Waverly 

 group, where it is represented, so far as known, by a shigle species; it reached 



70 



