70 



UINTACRINUS: ITS STEUCTUKE AND RELATIONS. 



younger specimens will give a good idea of the mode of increase, both by 

 upward growth and by intercalation, through which these variations are 

 produced. 



10 



Simpler forms of interbrachial areas, showing mode of increase of plates by intercalation and upward 

 growth. 3 and i are from specimen LVIIIA; .5 and 7 from a specimen in the Kansas University; 6, 8, 

 and 9 are from LVIII/; 10 is from No. 70. 



If we had still younger stages of growth we should no doubt find areas 

 with only one and two plates, just as we now find in the intersecundibrach 

 spaces. As it is, our lowest number — Fig. 3 — is three good-sized plates, 

 evidently developed from two in vertical succession — the third being pushed 

 in from the side, opposite the suture between them. In Fig. 4 a fourth 

 plate has been added, either from two, by lateral intercalation opposite the 

 suture from each side, or from three, by addition of a cuneate plate above. 

 In Fig. 5 a fifth plate is intercalated in the middle of the previous four, 

 while in Fig. 6 it is added at the top. In a similar manner a sixth, seventh, 

 and eighth plate is inserted, either at the top, in the middle, or both, — 

 Figs. 7, 8, 9, and 10, where the young plates are seen just beginning to form. 

 By a continuation of this process the more complex spaces are filled, up to 

 as many as 21 or 22 plates (PI. VI., Fig. 5). Where the increase takes 

 place as in Figs. 5, 7, and 10, the arrangement def?cribed by Clark is pro- 

 duced ; but where it is like that shown by Figs. 6 and 8, we get the form 

 of interbrachial space said to be characteristic of U. ivesifaliciis, and which 

 may be extended to areas containing 7, 8, or even 9 plates. 



Similar variations are found among the intensecundibrachs (interdistichals 

 audi.). Many specimens have two plates in a vertical row, as in U. tvesffalictis, 



