FROM THE UPPER GREENSAND. 97 



upper and under surface ; plates covered with a fine uniform granulation ; moutli-opening 

 situated in a concave depression. 



Dimensions. — Transverse diameter one inch and one tenth of an inch, height half an 

 inch. 



Description. — Although this Diadema exhibits a group of well-marked specific charac- 

 ters, its history, nevertheless, is involved in much confusion, from want of a careful 

 examination of the anatomy of the shell. 



The prominent ambital tubercles in the inter-ambulacra, their sudden diminution 

 in size on the upper surface, with the baldness of the test in that region, and the crowding 

 of the base with small tubercles nearly uniform in size, form a group of persistent 

 characters which distinguish Pseiidodiadema Rhodani from all its congeners. 



The smaller forms of this species were figured and described by Professor Agassiz as 

 Diadema Lucce, and the large tests as Diadema Bhodani. A series of specimens, of 

 different ages, has since shown that these two forms are identical. 



This initial error introduced the confusion that followed, and has rendered it a matter 

 of some difficulty to understand the synonyms of this species ; the careful study of a 

 good type form sent by the late M. Sgemann from the Gault (Etage Albien, d'Orbigny) 

 of Clars, near Escragnolle, department of the Var, has enabled me to compare our English 

 examples with an undeniable specimen, and from this examination to determine 'dwA.Diadema 

 Desori, Forb., and D. pustidatum, Forb., are different forms of Pseiidodiadema Rhodani. 

 My late esteemed colleague Dr. S. P. Woodward adopted Professor Forbes's materials in 

 his " Notes on British Fossil Diadems," contributed to Decade V of the ' Memoirs of the 

 Geological Survey ;' and it is evident from these notes that he had his doubts as to the 

 accuracy of our lamented friend's determinations, as will appear in the description of the 

 different species. 



There are two varieties of Pseudodiadema Rhodani — a large form, identical with the 

 type, figured by Agassiz,^ and a smaller form, corresponding with P. Lucce. The former I have 

 obtained from the Chloritic Marl of Chard ; the latter from the Upper Greensand of 

 Warminster, where it appears to be rare. The fine example figured in PI. XVIII, fig. 3, 

 a, h, c, is of moderate size ; the test is circular and depressed, slightly convex above, in- 

 flated at the sides, and very concave below ; the ambulacral areas are large, and a little 

 expanded at the sides to give increased space to the ambital tubercles ; from this point 

 they taper regularly towards both poles. There are two rows of primary tubercles, from 

 sixteen to seventeen in each, extending from the peristome to the disc ; three of these in 

 each row, at the ambitus, are large, and all those on the upper surface small, dimi- 

 nishing to mere granules near the disc (fig. 3 «); the tubercles on the under surface are small, 

 and have a uniform size to the peristome ; in this region the area is filled in with several 

 smaller secondary tubercles (fig. 3 b). The poriferous zones are slightly undidated at the 



1 'Description des Ecbinodermes fossiles de la Suisse,' tab. xvi, figs. 16 — 18, p. 9. 



13 



