NEW AGEI.ACRINITES 191 



stricting the family by the limits drawn by Bather and Jaekel,and 

 leaving out of present consideration the genus Edrioaster) 

 it is a notable fact that the rays are broadest and shortest in 

 the earlier species. This is very noticeable throughout the con- 

 siderable array of Siluric species, A. billingsi, cincin- 

 natiensis, holbrooki, dicks oni, etc. In the early 

 and middle Devonic (A. h a m i 1 1 o n e n s i s ; A. r h e n, a n u s , 

 upper lower Devonic, Unkel, Rhine) the rays have become 

 slender and very long^ but the most extreme expressions of 

 these characters are to be seen in the species of still later 

 age. It naturally ensues from this narrowing of the amibu- 

 lacra that the composition of these areas with the usual 

 preservation of the fossils becomes much obscured in later 

 forme. Billings claimed that in the Trenton species A. 

 dicksoni, perforated ambulacra! plates were exposed, but 

 this observation has not been confirmed and Jaekel holds that 

 no ambulacra! plates were present in these bodies. At all events 

 usually only the cover plates have been observed. The later 

 whiplash rayed species A. a 1 1 e g a n i u s, A. b e e c h e r i , A. 

 1 e b o u r i , A . ( E c h i n o d i s c u s) k a s k a s k i e n si i s , A . 

 (E.) s a m p s o n i , A. (L e p i d o d i s c u s) s q u a m o s u s, 

 show only rows of small, arched, angular cover plates more' 

 or less completely interlocking at their edges. 



Mouth and oral plates. The oral aperture is somewhat elon- 

 gated in all forms of true Agelacririites, and in some it 

 has been represented as covered with a few large oral plates. 

 This structure has, however, seldom been clearly made out 

 except for some of the Siluric species. In A. h a m i 1 - 

 t o n e n s i s . as shown in the accompanying figure of the type 

 specimen (pi. 10, fig. 6) it is somewhat schematic. Meek, on the 

 other hand, represents the oral region of A. s q u a m o s u s as 

 covered by a multitude of minute plates, but as no special men- 

 tion is made of the oral structure in his description, I infer that 

 this also is somewhat restored. In his description of E'chino- 

 discus and E. o p t a t u s, Miller makes special mention of the 

 fact that the rays do not depart from a central point, but that 

 this point of departure (oral opening) is elongated, two rays 



