86 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



In the Batocrinidge, 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 Batocrinidse, 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 bifurcate 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 Actinocrinidse, 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 Batocrinidse, 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 Bteganocriniis and Eiicladocrinus ; 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 RhodocrinidaB and Thysanocrinidse, 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 Steganocrinus scidptiis. Those of 

 Gilhertsocrinus are very delicate, and frequently pendent. The latter is the 

 case also in some species of Acrocrinus, and the Swedish genus Barrandeo- 



