70 The Ottawa Naturalist [October 



originated from a uniserial form, even in those cases in which 

 the arm structure at present is biserial, and diagrams are given 

 illustrating how a uniserial arm might develop into a biserial one. 

 It is well known that biserial arms frequently are uniserial at 

 the base, and the arrangement here is regarded as more primi- 

 tive. (See also Wachsmuth and Springer, Revision of the Pal- 

 aeocrinidea, II, 1881, pp. 22-25; III, sec. 1, 1885, p. 14; III, sec. 

 2, 1886, p. 230.) 



According to Austin H. Clark (A Monoograph of the Exist- 

 ing Crinoids, 1915, pp. 184, 189, 350, 352, 354), however, the 

 biserial arrangement is more primitive in crinoids; the biserial 

 arrangement being the palaeozoic type, while the uniserial 

 arrangement originated chiefly in post-palaeozoic times. 



Clark's conception of the origin of the biserial arrange- 

 ment of the ossicles of crinoid arms is so different fr m that 

 commonly accepted that it is quoted here in full: 



"The crinoid arms are primarily paired interradial struc- 

 tures which have become joined along their radial edges, forming 

 a radial biserial appendage, the ossicles later slipping in between 

 each other so that an elongate uniserial appendage results. The 

 original arms were, therefore, primarily ten in number. 

 Originally, before their union into five, the arms probably bore 

 no ventral ambulacral structures, and had no function other 

 than that of increasing the surface of the disk by increasing 

 the distance between the points of attachment." (Loc. cit., 

 p. 350.) 



The following statement by Clark also is illuminating: 



"In such fossil forms as have biserial arms it is to be re- 

 marked that at the arm bases the brachials become uniserial; 

 this is not to be interpreted as indicating that the arms were 

 originally uniserial, but quite otherwise; mechanical considera- 

 tions have forced the amalgamation of the two primitive radials 

 into one, and similarly have forced the uniserial arrangement 

 of the first two, and partially of the third and fourth, brachials." 

 (Loc. cit., p. 354.) 



"It is probable that the pinnules represent the original 

 type of crinoidal appendage, and that these appendages were 

 arranged in five pairs, the two components of each pair being, 

 so to speak, back to back; but the pinnules have become enor- 

 mously reduplicated, while in addition (they) have come to 

 lie along either side of long body processes (arms) of subsequent 

 development." (Loc. cit., p. 274, but omitting all references 

 to cirri.) 



Since the pinnules of crinoids are uniserial, it is certain 

 that Clark regarded the uniserial arrangement of ossicles as 

 primitive among crinoid appendages. Even the primitive arms 



