66 



UmTACRIKUS: ITS STRUCTUEE AND EELATIONS. 



the ratio of width to length from the young to the adult Antedon — from 

 lib ^^ ~W" — ^^ about the same as in our specimens, viz., about five times. 



An interesting illustration of these characteristics of the arm may 

 be seen in the photograph of specimen No. 233 (PI. VII., Fig. 4), which 

 shows, lying side by side, parts of six arms, belonging to as many different 

 individuals, of sizes ranging quite gradually from almost the maximum 

 to the minimum. The large arm begins at about the 25th brachial, and the 

 smallest one is almost at the distal end. The others are from median or 

 upper portions of the arm. Measurements of the brachials in these arm 

 fragments, taken in the order of their size beginning with the smallest, 

 are as follows : — 



^0 



1 . . 



. . length 



.50 mm. . . 



. width 



.60 mm. 





2 



a 



.75 '^ . . 



u 



1.10 " 





3 . . 



a 



1.00 '' . . 



(( 



1.50 " 





4 . . 



u 



1.15 " . . 



u 



2.00 " 





5 . . 



u 



1.25 '' . . 



cc 



2.25 " 





6 . . 



(( 



1.45 ^' . . 



(( 



4.50 " 



This gives about the same relative proportion as the data heretofore 

 given from different parts of the same arm. A similar result may be 

 arrived at by comparison of the brachials of specimens D 6, B 1, C 4, 

 and D 4, on the photographic Plate VIII. 



These specimens from Locality No. 2 also show, at first glance, a 

 marked difference from the others in the number of plates in the inter- 

 brachial spaces. A large proportion of them have but a few inter- 

 brachials, 5 or 6 plates being the rule among the smaller ones, — thus 

 giving the arrangement of U. westfalicus^ — whereas among the specimens 

 from Locality No. 1 the number is generally much larger. The variation 

 in this character, however, among the specimens from both localities is 

 so great, that I do not see how any specific distinction can be based upon 

 it. If U. westfalicus could be upheld as a distinct species, on account of 

 the arrangement of its interbrachial areas, then it might be that the 

 smaller Locality No. 2 specimens, although undoubtedly young individuals, 

 are not the young of U. socialis. In that case, with our present knowledge, 

 we should have to call them the young of U. westfalicus. 



I do not think it will be necessary to consider this contingency, for 

 reasons that will shortly follow. For an attentive comparison of the 

 facts exhibited by all the specimens will show that the variation in the 



