MORPHOLOGICAL PART. 85 



The structure of this species is a very strong indication that the arm 

 branches are modified pinnules ; and this suggests that non-pinnule-bearino- 

 brachials, whether fixed or free, may form a syzygy whether the succeeding 

 axillary supports an arm or a pinnule. Carpenter pointed out ^ that among 

 recent Crinoids, with but few exceptions, " the first two joints beyond every 

 axillary of the dividing rays are united to one another in the same man- 

 ner, either by syzygy or bifascial articulation." This rule seems to hold 

 good also for most of the Palseozoic Crinoids, with the exception that we 

 find among them only syzygies, and no articulation. 



The Actinocrinid^, as we have stated, represent an advanced stage of 

 the Camerate type. Not only their costals, but also their distichals, and 

 generally several more orders of brachials, participate in the formation of 

 the calyx, and all the branches are given off alternately from opposite 

 sides of the main trunks. In the genus Actmocnmis, t only every second 

 or third brachial of each order in the calyx supports an arm (Plate LX., 

 Fig. la); but in Cactocmms, F/i?/setocrmus, Teleiocriniis, and Strotocrinus, an 

 arm originates from each successive plate (Plate LXY., Figs, la and Ic), and 

 the arms alternate like the pinnules. In AcUnocrinus occasionally, and in Am- 

 plioracrimis generally, branches are also given off at irregular intervals from 

 the free arms; but these evidently made their appearance after the arms 

 had become biserial. In Sieganocrimis the branching is continued to the top 

 of the rays, as in Eudadocrinus (Plate LXI., Figs, la and Id), and each order 

 of brachials consists of one, two or three plates, which are formed into tubular 

 appendages of the calyx, giving off biserial, pinnule-bearing arms. Accord- 

 ing to our interpretation, we have in AcUnocrinus and Steganocrinus a series 

 of syzygies, in the former extending to the top of the calyx, in the latter to 

 the top of the rays ; as opposed to Cadocrinus, Strotocrinus, etc., in which 

 each order of brachials consists of a single plate, which is axillary. 



The pinnules of the Actinocrinidse differ from those of other families in 

 being provided with prominent hooks, directed obliquely upward and out- 

 ward. These hooks are arranged in rows parallel to the sides of the arms, 

 and those of one pinnule overlap the corresponding ones of adjoining pin- 

 nules (Plate LVIII., Figs. 1 and la, h, c), so as to give to the mass of pinnules 

 the appearance of a highly complicated network. 



* Chall. Rep. I, p. 49. 



t We subdivide the genus AcUnocrinus, as heretofore recognized, into AcUnocrinus proper, and Cacto- 

 crimis ; referring to the former only the lobed species with two or more bracliials to each order ; and to the 

 latter those in which the free arms are arranged equidistant around the calyx, and each order of brachials 

 above the costals consists of but one plate. 



