92 rSEIDODIADEMA^ 



DiPLOPODiA Mai-BOSi, Dujardvi ct Hupe. Hist. Nat. des Zooph. Echino- 



Jermes, p. .501, 1862. 

 PsEUDODiADEMA Malbosi, Cotteau. Kcliinid. Foss. des Pyrenees, p. 26, 1863. 



— — Cotteau. Paleontologie Fran9aise, Ter. Cretace, torn, vii, 



p. 448, pis. 1106 et 1107, 186.1. 



Test large, subcircular, upper surface convex, slightly inflated, base rounded and 

 flattened, ambidacral areas narrow, contracted at the upper part by the width of the pori- 

 ferous zones, two rows of tubercles twenty to twenty-two in each row. Inter-ambulacral 

 areas wide with four, six, or eight rows of tubercles at the equator, the two inner rows 

 having eighteen to twenty tubercles in each, extend from the peristome to the disc, all 

 the others disappear at diff'erent points on the sides. Small secondary tubercles 

 scattered irregularly among the primary series in the inferior part of the areas. Poriferous 

 zones narrow at the base and sides ; pores in double file from the ambitus to the disc, 

 Avhere they increase in width, and. on the upper third are largely bigeminal. Mouth- 

 opening moderate in size ; peristome nearly equal lobed ; discal opening large and 

 acutely pentagonal. 



Dimensions. — Transverse diameter two inches ; height thirteen twentieths of an 

 inch. 



Description. — This is a very rare British Urchin, and as nearly all the tests have 

 been either broken, crushed, or otherwise distorted, it is difficult to form a correct 

 idea of its form. I have carefully examined the original specimens collected by 

 Mr. Mackeson, F.G.S., from the Lower Greensand at Hythe, and presented by him to 

 the Royal School of Mines ; these I have compared with a series collected by my 

 friend the Rev. T. Wiltshire, from the Lower Greensand at Whales' Chine, Isle 

 of Wight, with which they agree, and both correspond with the figures and descrip- 

 tion of Pseudodiadema Malbosi given by M. Cotteau in the 'Paleontologie Franjaise,' 

 and with a good type specimen kindly presented to me by j\L Bayle, of the Ecole 

 des Mines, Paris. I have no hesitation, therefore, in considering D. Mackesoni, Porb., 

 identical with D. Malbosi, Agass. It is important likewise to note that both belong to the 

 same geological horizon ; the French specimens were collected from the Upper Neoco- 

 mian, associated with jEcliinospatagus Collegnii, Sism., and the British specimens from 

 the Lower Greensand at Ilythe, and the Crioceras-beds, Lower Greensand, at Whales 

 Chine, Isle of Wight, the English equivalent of the Continental Neocomian formation. 



This Urchin attains a considerable size ; Mr. Wiltshire's cabinet contains a specimen 

 measuring two and a half inches diameter. The base of this fossil is nearly circular, and 

 only slightly pentagonal. In some of the Ilythe specimens in the Museum of the Royal 

 School of Mines, the upper surface is convex and moderately inflated, and the ba'^e 

 rounded and flattened. 



The ambulacral areas are narrow and contracted at their apices by the width of the 

 poriferous zones above (fig. \,(j)\ they are slightly inflated, and furnished with two 



