UINTACRmUS: ITS STEUCTURE AND RELATIONS. 67 



number of interbrachial plates is also an individual one, due to growth. 

 *As these are supplementary plates, whose office is to fill up the space 

 required for the expansion of the calyx, we should expect to find that, 

 as a general rule, their number increased with age, — and such proves to 

 be the case. This will be more fully considered under the next head. 



RELATIONS OF U. WESTFALICUS TO THE AMERICAN SPECIMENS. 



The only differences that have hitherto been pointed out between 

 U. westf aliens and U. socialis are : — 



1. That the brachials in U, socialis are decidedly broader than in 



U, westfalicus, 



2. That the interbrachials differ in number and arrangement. 



As to the first, by referring to Table D supra, containing measurements 

 of brachials in various individuals, it will be seen that this difference may be 

 readily traced to difference in age of the individual. The specimen figured by 

 Schlueter * would be of about the medium size of U. socialis as found by me. 

 The calyx is about 41 mm. wide, and the brachials at about the 10th IIBr. 

 are 5 mm. wide, and about 1.37 mm. long; or the width is about three 

 times the length (PL III., Fig. 7). This is about the proportion of medium- 

 sized specimens of U. socialis, — as, for instance, my No. 3 (PL V., Fig. 2), 

 which with a calyx 39 mm. wide has the corresponding brachials 3.75 mm. 

 wide and 1.22 mm. long ; while if the comparison were made with young 

 specimens (PL V., Fig. 4), it would appear that the brachials of U. 2vestfalicus 

 are much the broadest of the two. 



The second point requires careful consideration, as it has been chiefly 

 relied upon by Schlueter and the authors subsequent to him to distin- 

 guish the two species. Neither Grinnell nor Schlueter seem to have 

 understood the nature of the fixed pinnules, for they treat the plates 

 composing them as ^^ interradials " and " interaxillaries," without distin- 

 guishing them from the plates lying below them, which properly belong 

 to those categories. Condensing Schlueter's description as to these plates, 

 it amounts to this : — 



* Zeitsch. d. Deutsch. Geol. Gesel., XXX., Taf. IV., Eig. 1. 



