Dr. J. W. Gregory on Pseudodiadema Jessoiii. 465 



LXVITL — Pseudodiadema Jessoni, sp. n., an Echinoid from 

 the English Oxford Clay. By J. W. GREGORY, D.Sc, 

 F.G.S. 



Pseudodiadema Jessoni^ sp. n. 



Diagnosis. — Test small, high, tumid. Aboral lialf slightly 

 more conical than the lower. 



Apical system fairly large, less than half the diameter of 

 the test, pentagonal. 



Ambulacra narrow. (The plates are normal, i. e. simple 

 primary plates near apical system and compound plates of 

 three primaries at ambitus.) Two rows of small tubercles 

 down each ambulacrum. 



Inter ambulacra. — Seven or eight plates in each series in 

 the adult. The plates are high ; each of the five middle 

 plates lias a single large tubercle upon it, occupying nearly 

 the whole of the plate. The uppermost plate has no tubercle ; 

 the uppermost tuberculate plate has a complete scrobicular 

 circle; in the remainder the scrobicular areas are confluent. 

 The granulation occurs mainly as a narrow band along the 

 middle of the interambulacrum, but it is very limited in 

 amount. 



Peristome large, circular ; deep branchial clefts. 



Dimensions o/'E 3937 : — 



millim. 



Diameter 14 



Height 8 



Diameter of apical area 6 



„ peristome 8 



Width of ambulacrum at ambitus 2-2o 



,, interambulacrum at ambitus 6*25 



Numljer of pore-pairs in one series 25-26 



Number of interambulacral plates in one series . . 7 or 8 



Distribution. — Oxford Clay, St. Ives, Hunts. Collected 

 by T. Jesson, Esq. 



B.M. type E 3936; larger specimen E 3937. 



This small Echinid is of interest as the only species as 

 yet recorded from the Oxford Clay of this country. The 

 British Museum collection contains twenty specimens which 

 were found by Mr. Jesson, and there are others in the Wood- 

 wardian Museum at Cambridge. The species is characterized 

 especially by its high unituberculate plates, thick form, 

 confluent scrobicular areas, and the sparseuess of the granules. 



The large size of the apical area suggests at first that the 



