190 



NBW YORK STATE MUSEUM 



Fio. 3 A. dlckaoni Billings 

 X 1^ showing the five contra- 

 solar rays and the ambulacral 

 plates which were regarded as 

 perforate by Billings ( From 

 Ottawa Held nat. club. Trans. 

 2. ISSl. plate flg. ^) 



or all contrasolar, as in A. d i c k s o n i Billings (Trenton). 

 Four rays contrasolar and one solar is the usual expression, 

 as shown by A. ( L e p i d o d i s e u s ) 

 s q u a m o s u s Meek and A. ( E e h i - 

 nodiscus) kaskaskieneis Hall 

 of the Keokuk and Chester groups 

 of the lower (''arboni<-. A. cin- 

 cinnatiensis Koemer, A. h o I - 

 b r o o k i and A. p i 1 e u s Hall of the 

 Cinciiinatian group. In A. h a m i 1 1 o n - 

 e n s i s Vanux. (middle Devonie) two ai-e 

 solar and three contrasolar. Even the 

 number of the rays seems not to be always 

 live, as Faber has de^ribed a species sup- 

 posed to have seven rays, (A. s e p t e m - 



brachiatus; Cincinnatian) and Miller and Gurley one with 

 but four (A. legrandensis; Kinderhook). 



Young specimens of A. a 1 1 e g a n i u s, A. b u 1 1 s i and of the 

 A. h a m i 1 1 o n e n s i s show that the ambulacral rays in early 

 growth extended in direct radial lines to the margin or elevated 

 submarginal wall. They did not however pass on to the aboral sur- 

 face, though in A. a 1 1 e g a n i u s the^* reach the margin, but in 

 the young of A. h a m i 1 1 o n e n s i s and A. b u 1 1 s i these rays 

 abut dii'ectly against a highly developed ridge. The final course 

 of the rays is then not determined except with the appixxich of 

 mature conditions, but is nevertheless constant, and we have no 

 record of any departure from the I'egularity and uniformity of 

 their direction in a given species or homogeneous group of 

 species.' 



This feature is notably one in which specific character is not 

 expr(*ssed or suggested before the commencement of mature 

 growth, and it seems therein to lose all value as a feature of 

 higher (generic) distinction, though persistent as a specific 

 character. The primitive direction of these i*ays is repeated in 

 the adult expressions of the earlier agelacrinites, as cited above. 



iSizCf length and striicture. Among the Agelacrinitidae (re- 



