466 Dr. J. W. Gregory on Pseudodiadema Jessoni. - 



Echinid may be an Acrosalem'a ; but the structure of the 

 ambulacral plates near the peristome shows that it belongs 

 to a different family. In spite of the absence of the apical 

 system there is no doubt that it is a Pseudodiadema. 



Fig. a.— Test of E 3936, X f| diam. 



Fig. b. — An ambulacrum and interambulacnim, X 5^ diam. 



Its closest ally is Pseudodiadema Beaudouintj Cott.'^, from 

 the Upper Oxfordian of Chatillon-sur- Seine, Cote d'Or, 

 which is said to be very rare. From this the English 

 species differs by being much higher in proportion to its 

 width ; the ratios in the new species are 1 : 3, and in P. Beau- 

 douini 1:2; the number of interambulacral plates is smaller, 

 the granules are less abundant, there is no tubercle on the 

 uppermost interambulacral plate, and the pore-pairs near the 

 peristome are in a straight single series. 



The character of the granulation, the number of inter- 

 ambulacral plates, and the size of the interambulacral plates 

 are points which the new species has in common with Pseudo- 

 diadema Lavgi, Desor f ; but from this it differs by the large 

 size of the ambulacral tubercles and the crowding of the pore- 

 pairs near the peristome in the continental species. The 

 same characters separate it from Pseudodiadema priscum 

 (Ag.) :j: and Pseudodiadema drogiacurn § (Cott.), with both of 

 which it has resemblances in some details. 



* G. Cotteau, Pal. fran^., Terr, jurass. t. x. pt. ii. ; £chinides r^guliers, 

 livr. 7, 1882, p. 287, pi. cccxxxviii. figs. 1-5. 



t See especially figures by Desor and de Loriol, EcLinol. helvet. 

 pi. xxiv. fig. 2. 



X L. Agassiz, Cat. Syst. Ectyp. foss. Ech. Mus. Neocom. 1840, p. 8. 

 Figured e. g. by Desor and de Loriol, op. cit. pi. xxiv. fig. 3 ; Cotteau, 

 oj}. cit. pi. cccxxxv. 



§ Cotteau, Et. Ech. Youne, p. 156, pi. xix. figs. 6-10, and oj). cit. 

 pi. cccxliii. figs. 7-13. 



