86 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
In the Batocrinidx, we find an arm structure such as occurs in no other 
group. While in all other Camerata the ambulacral openings of the calyx 
give origin to but a single arm, those of the Batocrinites frequently bear two, 
either from every opening or from any less number of them, and this, which 
is very remarkable, even in the same species. Thus the specimens may have 
eighteen arm openings and but eighteen arms, or any number from eighteen 
to thirty-six. This mode of multiplying the number of arms is found only 
among the later forms, and it is evident from the structure that the second 
arm, where it occurs, was introduced in the nearly mature individual after the 
arms became biserial, and the lower orders of brachials had been incorporated 
into the calyx. The bifurcation takes place in the same manner as in free 
biserial arms, there being no true axillary, and the plate taking its place 
is no larger than the succeeding arm pieces. 
In the Batocrinide, all branching in the calyx is by means of dichotomy, 
and the number of arm openings in the three anterior rays rarely exceeds 
four; but there may be only two or three, and in the posterior rays, the 
arms next to the anal interradius may bifureate once or twice again, so that 
two of the rays have five or six arm openings. In some genera the number 
of arm openings varies considerably among the rays, the anterior, or occa- 
sionally the antero-lateral ones, being generally the least developed. This is 
not the case in the Actinocrinide, in which normally, with one or two excep- 
tions, the number of arms is equally divided among the rays. In the Peri- 
echocrinites, which we made a subdivision of the Batocrinide, the free arms 
almost always branch in their biserial stage ; while they remain simple in 
the Batocrinites. 
In Melocrinus a very peculiar arm structure occurs, somewhat similar to 
that of Steganocrinus and Fucladocrinus; but while in the latter the two divi- 
sions of the ray form independent appendages, all the way from the calyx 
up, those of Melocrinus, either for some distance or to the full length ot the 
rays, are laterally connected, and form together but one appendage, from 
which the arms are given off at intervals from opposite plates in the ray, 
and from one side only of each half. 
In most of the Rhodocrinidse and Thysanocrinidx, the arms branch in 
their free stage, whether uniserial or biserial. The arms of Ripidocrinus are 
given off at both sides of the main rays, as in Sfeganocrinus sculptus. Those of 
Gilbertsocrinus are very delicate; and frequently pendent. The latter is the 
case also in some species of Acrocrinus, and the Swedish genus Barrandeo- 
