68 THE CRINOIDEA CAMERATA OE NORTH AMERICA. 



between Merocrimis and locrinus, and between Ottawacrinus and Heterocrinus, — 

 dicyelic and monocyclic forms respectively. 



While the evidence thus points to the probability that the one group was 

 derived from the other, it is difficult to explain the change in the orientation 

 of the stem. That the whole crown passed through a revolution of 36° is 

 highly improbable. The change probably took place in the body of the 

 stem, modifying its form to suit the condition of the base. The stem is 

 round in most of the earlier Crinoids ; but we find occasionally within the 

 same family quadrangular stems from a quadripartite base, together with 

 pentangular stems from a quinquepartite base, which goes to prove that the 

 stem, to some extent at least, adapted its form to the conditions of the base. 

 In a similar manner interradial stems may have changed into round stems, 

 and these into radial stems. This, however, does not explain the change 

 in the orientation of the canal, and that of the cirri. The latter are 

 directed to the angles of the axial canal, through which they communi- 

 cate with the chambered oro^an. 



The only writer, besides Bather, who has discussed the derivation of the 

 two groups is Dr. J. Walther,*^ who takes it for granted that the simpler — 

 monocyclic — form is the ancestral one. His arguments were suggested from 

 his study of the pentacrinoid larva of Antedon, and are extremely hypo- 

 thetical. They are based upon his peculiar interpretation of the basals and 

 infrabasals, which, as he now will himself admit, cannot be upheld since Bury 

 discovered infrabasals in the larva of Antedon. 



C. The Badials, 



The term ^-radials" is applied by us only to the first plate of each ray. 

 All succeeding plates in a radial direction, whether free or incorporated into 

 the calyx, are brachials. The name was given by Miiller to all plates up 

 to, and including the first axillary, and all pieces beyond were regarded by 

 him as armplates. The same view was taken by Roemer, de Koninck, and 

 other European authors. The American Palasontologists applied the term 

 not only to the plates of the first order, but to all plates of the rays that take 

 part in the calyx. They distinguished those of different orders as primary, 

 secondary, and tertiary radials, according to rank. 



* " Untersucliiiiigeii iiber den Bau der Crinoideen." Palaeontographia, 1866, Bd. XXXH., pp. 180- 

 199. 



