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-bearing 
brachials, whether fixed or free, may form a syzygy whether the succeeding 
axillary supports an arm ora 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 Palzozoic Crinoids, with the exception that we 
find among them only syzygies, and no articulation. 
The Actinocrinidx, 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 Actinocrinus, + only every second 
or third brachial of each order in the calyx supports an arm (Plate LX., 
Fig. la); but im Cactocrinus, Physetocrinus, Teleiocrinus, and Strotocrinus, an 
arm originates from each successive plate (Plate LXV., Figs. la and 1c), and 
the arms alternate like the pinnules. In Acéinocrinus occasionally, and in Am- 
phoracrinus 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 Steganocrinus the branching is continued to the top 
of the rays, as in Hucladocrinus (Plate LXI., Figs. la and 1d), 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 Actinocrinus and Sleganocrinus 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 Cactocrinus, Strotocrinus, ete., in which 
each order of brachials consists of a single plate, which is axillary. 
The pinnules of the Actinocrinidx 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 LVIIL., Figs. 1 and 7a, b,c), so as to give to the mass of pinnules 
the appearance of a highly complicated network. 
* Chall. Rep. I., p. 49. 
¢ We subdivide the genus Actinocrinus, as heretofore reeognized, into Actinocrinus proper, and Cucto- 
crinus ; referring to the former only the lobed species with two or more brachials 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. 
