68 



UmTACKINUS: ITS STEUCTUKE AND KELATIONS. 



The interradials (iriterbrachials of current terminology) consist of five 

 large plates (sometimes six), which generally project like an arched shield $ 

 these vary in outline and position in the different interradii. The interdis- 

 tichals (intersecundibrachs) are two, lying one above the other. 



Comparing the two species, he says : — 



" In form and size the two crinoids show no other differences than that in 

 U. socialis the interradii do not project. . . . The most important observable 

 difference on which a specific distinction can be based lies in the variation 

 of the intermediate plates," — i. e. the interradial and interaxillary fields.* 



Clark, in discussing the relations of the two species, says : — 



" In the number and arrangement of the interradials the most marked 

 difference is manifested. In U. socialis seven interradial plates encircle the 

 eighth, or eighth and ninth, as the case may be ; while in U. wesifalicus the 

 interradials are fiYQ in number, all of which come in contact with plates of 

 other areas." t 



Bather says : " It is chiefly in the arrangement of the interradial plates 

 that this species differs from fT". socialist X 



With regard to the projection or swelling of the " interradial " areas, 

 this occurs occasionally among my specimens. No. 45 (PL III., Fig. 2) 

 shows it very conspicuously, and it is found in about ten others. On Slab I 

 there is a specimen with arched interradii, in which two of them contain but 

 a single large plate, formed by the anchylosis of five or six original plates. 



It is not difficult to pick out from among the specimens of U. socialis in 

 this collection some which are in all essential points almost the exact coun- 

 terparts of Schlueter's specimen. No. 242 (PL IIL, Fig. 6), in size of calyx, 

 proportions of brachials, number of interbrachials and intersecundibrachs, is 

 substantially identical with it, and No. II h and 247 also, except in the base, 

 as the following comparison shows : — 





Calyx. 



iBr. 



illBr. 



Base. 



U. westfalicus . 



41 mm. 



5-5 



2 



M 



No. 242 ... 



44 " 



5-5 



2-1 



M 



" lib . . . 



50 '' 



5-5 



2- 



D 



" 247 .. . 



41 '^ 



3-5 



2-3 



D 



The difference in shape of the calyx is incidental, due to the manner 

 of compression. 



* Op. cit. p. 61. t Op. cit. p. 23. J Op. cit. p. 977. 



