no. 1636. ARM HOMOLOGIES IN RECENT CRINOIDS— CLARK . J25 



attention to two points of interest. In Uintacrinus (fig. 25), which 

 is most nearly related to the Comasteridse, the peculiarities of the 

 pinnulation" are at once explained if we consider Z x and Z 2 to be 

 the third and fourth joints after the axillary, instead of the first and 

 second, as would be expected; moreover, the size and the shape of 

 the joints and the examination of the external lines of contact of the 

 articulations lead us to the same conclusion, while I have already 

 shown h that the abnormalities re- 

 corded by Mr. Springer in his mono- 

 graph of the genus again favor this 

 interpretation. The arms of Uinta- 

 crinus, therefore, after the costal 

 axillary, resemble those of Eudiocrinus 

 in having a repeated Z x and Z. 

 series of which the second is not an 

 axillary. I have already c called 

 attention to a similar state of affairs 

 occurring abnormally in a specimen 



# & J # L Fig. 25. — Uintacrinid.e ; UintacrI; 



of Heliometra tanner/. It was stated nus (adapted from Springer) ; 

 that muscular articulations were oc- XHB " ^ierradial " and inter- 



BRACHIAL PLATES ARE OMITTED SO 



casionally divided, so that an axil- A s to more clearly bring out 

 lary was formed giving rise to a the arms am. pinnules. 

 pair of arms instead of to a single arm. The thought naturally 

 arises, does the straight muscular articulation on the distal face of 

 the radial ever divide; and do the oblique muscular articulations of 

 the distal part of the arm ever divide? In answer to the first ques- 



£ , — — — — - — 



a In Uintacrinus the first pinnule is on the second post-axillary joint, the next 

 on the fourth, and on the opposite side of the arm. Now. these pinnules are 

 separated by two articulations. Wore they both muscular, they would, so far 

 as the position of the pinnule is concerned, counteract each other, and the 

 second pinnule would be on the same side as the first; were they both non- 

 muscular neither would have any effect on the pinnulation. and the second pin- 

 nule would again be on the same side as the first : but it is on the opposite side; 

 therefore, one of the articulations must be muscular, and the other nonmuscular. 

 A pinnule can not be developed at a nonmuscular articulation; therefore, the 

 articulation at the distal end of the second post-axillary joint is muscular; 

 hence the articulation between the third and fourth post-axillary joints must 

 be nonmuscular, either a synarthry or a syzygy. In the comatulids, the pin- 

 nule on Z 2 is almost universally different from that on all succeeding brachials, 

 but resembles those on all the interpolated repetitions of Z». In Uintacrinus 

 the second pinnule resembles the first, and not those following (in size) : hence, 

 the conclusion is reached that the joint which bears the second pinnule is 

 homologous with that which bears the first, and that the first and second post- 

 axillary joints in Uintacrinus are an interpolated Zi Z 2 series, of which the 

 second is not, as is usually the case, an axillary. 



b Proc. U. S. Nat. Mus., XXXIV, p. 269. 



c Idem., XXXIV, p. 2G7. 



