106 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



of all the outermost axillaries), an arm and a division series, or, abnormally but 

 not infrequently, two exactly similar pinnules. 



While ordinarily a definite and fixed pair of structures is found on the 

 distal end of the distal element of any given pair of ossicles united by nonmus- 

 cular articulation, these pairs of structures are more or less interchangeable, and 

 such interchange is a common source of variation. 



In Antedon bifida and in Leptonemaster venustus nine-armed specimens have 

 been recorded in which, through variation, one of the post-radial series was un- 

 divided, but possessed at the base two exactly similar pairs of brachials. In these 

 cases it is evident that the normal IBr series were actually present, but the IBr 2 , 



instead of being axillary 

 and carrying two arms on 

 the distal border, carried a 

 single arm and a pinnule. 



Thus through varia- 

 tion resulting in the re- 

 placement of one of the two 

 arms normally following a 

 IBr series by a pinnule, 10- 

 armed species may on one 

 ray occasionally assume the 

 brachial structure charac- 

 teristic of Eudiocrinus. 



The 10-armed comatu- 

 lid genus Uintacrinus (fig. 

 147. p. 83) has a peculiar 

 arm structure, at first sight 

 apparently differing from 

 that of any other comatulid. 

 The arms divide once, upon 

 J87 the second postradial os- 



Fio. 187. CENTRAL PORTION AND ONE POST-RADIAL SERIES OF TYPE side The first pinnule is 



on the second postaxillary 



ossicle on the outer side of the arm; the second on the fourth, on the inner side; 

 the third on the fifth, on the outer side; and the following pinnules occur alter- 

 natingly on succeeding brachials. The third and fourth postaxillary ossicles 

 resemble the first and second in size and in the direction across the arm of the suture 

 line between them, and in these features differ from the ossicles succeeding. The 

 second pinnule is more like the first than like the third. 



The absence of pinnules on the first and third postaxillary ossicles, the 

 similarity between the pinnules on the second and fourth postaxillary ossicles, 

 the similarity between the third and fourth and first and second postaxillary 

 ossicles, and the similarity in the direction taken by the joint lines between the 

 third and fourth and first and second postaxillary ossicles, leave no room for 



