78 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



ous, are the costals of Dendrocrmus and Homocrinus, in which we have counted 

 as many as seven ; locrinus has four, A^iomalocrimis from two to four in the 

 same species. Most of the other Fistulata have one or two. When there is 

 but one jDlate, it is generally twice as long as the two, the latter forming 

 ^ ^y^JEJ' We find this in the majority of the Poteriocrinida3, 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 Poieri- 

 ocrinus missouriensis from ten to fourteen. In the Hybocrinidse, and in the 

 Larviformia as a rule, the arms are formed exclusively of costals (Fig. 1); 

 and only occasionally in Allagecrinus, 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: 

 Steganocrifius sculpius, a few species of Melocrims, Hyocrimis and Calamocrinus, 

 have no regular distichy, and all their branches are given off from one 

 trunk. A similar structure is found among the Poteriocrinidse 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 Actinocrinidae. 



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 



