NEW AGELACRINITES 



189 



the larger plates may themselves be peripheral and project down- 

 ward below the level of the aboral surface (A. alleganius) 

 or all the marginal plates be of uniform and small size 

 (A. cincinnatiensis), with the peripheral area outside 

 the circular wall very broad (A. b u 1 1 <s i , A. legrandensis) 

 or these plates be both large and small (A. dicksoni). 



Agelacrinites holbrooki James of the upper Siluric 

 has the inteirradial spaces paved with a mosaic of five and six 



sided plates, while the marginal 

 plates are imbricating. 1 This condi- 

 tion, though pronounced in this spe- 

 cies, is also apparent in less degree 

 in all species with mosaic plates over 

 the interradii. Such a combination 

 of plating would seem essentially to 

 neutralize the influence of the char- 



Fio. 2 A. holbrooki James. Anal acter of the thecal plate® alone aS a 

 region much enlarged, showing the r 



mosaic plates of the interradii, the generic feature, when considered in- 



squamous plates of the margin, the 



crowding of smaii plates about the anai dependently of other structure. It 



pyramid and the terminations of K 1 



and 5. Between the cover plates of was the difference in this respect 



the latter will be observed minute 



accessory plates, two for each interval, indicated by the first species above 



mentioned, together with a difference 

 in the attitude of the rays, that furnished to Meek occasion for 

 introducing his term, Lepidodiscu®, for the squamous forms. 



Madreporite. Dr Bather has indicated the presence of a madre- 

 pore in his copied figures of A. cincinnatiensis and A. 

 h a m i 1 1 o n e n;s i s (op. cit. p. 205). I have, however, seen 

 nothing in any agelacrinite that can be safely thus designated* 



Rays. Direction. Of the species of "Agelacrimus ", some of 

 the early Siluric forms like A. billing s i Chapman of 

 the Trenton and A. bohemicus Barr. (fitage D), have 

 the rays sharp and quite straight, abutting against or 

 tapering to a broad margin of larger and smaller plates. 

 In others the rays are all isolar, as in A. alleganius, 



1 Cin. soc. nat. hist. Jour. 1887. 10 : 25. 



