466 Dr. J. W. Gregory on Pseudodiadema Jessoui. 



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 ^ diam. 



Fig. b. — An ambulacrum and interambulacrum, X 5| diam. 



Its closest ally is Pseudodiadema Beaudomni, 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- 

 domni 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 ))lates, and the size of the interambulacral plates 

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

 diadema Latifft, Desorf ; 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. TJie 

 same characters separate it from Pseudodiadema priscum 

 (Ag.) I and Pseudodiadema drogiacum § (Cott.), with both of 

 which it has i-esemblances in some details. 



* G. Cotteau, Pal. franf., Terr, jurass. t. x. pt. ii. ; Echiuides rt5guliers, 

 livr. 7, 1882, p. 287, pi. cccsxxviii'. 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. h^ Desor and de Loriol, op. eit. pi. xxiv. fig. 3 ; Cotteau, 

 0}}. cit. pi. cccxxxv^ 



§ Cotteau, Et. Ecli. Youue, p. 156, pi. xix. figs. 6-10, and op. cit. 

 pi. cccxliii. figs. 7-13. 



