78 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
ous, are the costals of Dendrocrinus and Homocrinus, in which we have counted 
as many as seven; Jocrinus has four, Anomalocrinus from two to four in the 
same species. Most of the other Fistulata have one or two. When there is 
but one plate, it is generally twice as long as the two, the latter forming 
a syzygy. We find this in the majority of the Poteriocrinide, except in 
the anterior ray, which in some species has as many as twelve costals, while 
in others it has no bifurcation at all, and the arm is composed of costals only. 
A few of their species have from five to eight costals in each ray, and Poteri- 
ocrinus missouriensis from ten to fourteen. In the Hybocrinide, and in the 
Larviformia as a rule, the arms are formed exclusively of costals (Fig. 1); 
and only occasionally in Ad/agecrinus, in one or two of its rays, the radials 
are axillary and in the absence of costals support two rows of distichals. 
From these facts it is obvious that the number of costals does not consti- 
tute a reliable character for classification, as heretofore supposed, and that in 
some groups their number is of but little value for specific distinction. This 
is even more markedly the case with regard to the higher divisions of the rays. 
The distichals are borne upon the axillary costal, which splits the ray 
into two divisions, and all succeeding bifurcations take place from one or 
both of these divisions. To this rule, however, there are a few exceptions: 
Steganocrinus sculplus, a few species of Melocrinus, Hyocrinus and Calamocrinus, 
have no regular distichy, and all their branches are given off from one 
trunk. <A similar structure is found among the Poteriocrinid in the pos- 
terior ray. In most of their species with two arms to the ray the posterior 
ray has but one trunk, and in multibrachiate forms the first bifurcation of 
the posterior ray corresponds with the second in the other rays. Branching 
takes place either alternately from opposite sides, or by means of dichotomy. 
The former is very frequently the case among the Camerata, and is the rule 
in the Actinocrinide. 
The arms are composed of one or two rows of plates. The uniserial 
arms are composed of either rectangular or cuneate plates, the former being 
the most archaic form. The cuneate plates are alternately arranged, and 
gradually pass into a biserial arrangement. Arms are called “ biserial” 
when the plates interlock, and do not reach to the full width of the arm. 
This explains why in biserial arms the pinnules are given off from every 
plate at each side of the arm, while in uniserial arms every second plate at 
each side bears a pinnule. 
That the biserial arms represent the higher form is clearly shown by 
