FROM THE LIAS. Ill 



upon sure ground when endeavouring to distinguish the affinities and differences existing 

 among them. After a diligent search for urchins in the Lias of Gloucestershire, I have 

 succeeded in collecting from these rocks only a very few examples of this group. In addi- 

 tion to those found near Cheltenham, my friend Mr. Moore, of Bath, kindly presented me 

 with a few specimens which he collected from the Upper Lias near Ilminster. and from 

 these collective materials the species under consideration was discovered. 



Pseudodiadema Mooreii has a circular outline, slightly inclining to a pentagonal contour ; 

 it is much depressed at the upper surface, and is flattened at the base (tig. 1 a). The ambu- 

 lacral areas are very narrow, being less than one third the width of the inter-ambulacral 

 (fig. 1 b) ; their margins are occupied by two rows of tubercles, about eight in each row 

 (fig. 1 d), which, at the base, and up to the equator, are nearly as large as those of the 

 inter-ambulacra, but from that, region to the apex of the area they rapidly diminish in size, 

 and are here very disproportionate in magnitude to them ; a zigzag line of single granulation 

 separates the two rows of tubercles from each other (fig. 1 d). The inter-ambulacral 

 areas are wide and well developed (fig. 1 b, c), from eight to nine in each row, which 

 occupy the centre of the plates (fig. 1 a) ; the areolas of the tubercles on the upper surface 

 are surrounded with a circle of granules which separates them from each other (fig. 1 d), 

 but those of the base are confluent above and below (fig. 1 c). The miliary zone at the 

 base of the test has a number of granules scattered over it (fig. 1 e) ; whilst on the upper 

 surface the plates are destitute of any other ornament beyond the faint circles that 

 surround the tubercles (fig. 1 b). 



The poriferous zones are narrow, the pores are strictly unigeminal (fig. 1 b, d); the 

 avenues are, however, rather flexuous below ; the basal tubercles of both areas are nearly 

 alike in size, but on the dorsal surface those of the ambulacra dwindle into large granules ; 

 whilst those of the inter-ambulacra maintain their size up to the last pair, which are small 

 near the margin of the disc. The mouth opening is large (fig. 1 e), and the peristome is 

 divided into ten nearly equal-sized lobes. 



The apical disc is partly preserved in the specimen here figured (fig. 1 b). It consists 

 of five large ovarial plates, of a heptagonal form ; two of the sides unite with the inter- 

 ambulacral plates, two with the ocular, two with the adjoining ovarials, and the single 

 surface contributes to form the boundary of the anal opening, which is of moderate size; 

 the five ocular plates are small and heart-shaped ; their apex is directed towards the anal 

 opening, and their base to the area; the madreporiform tubercle is slightly elevated on the 

 right antero-lateral ovarial plate ; the surface of all the discal elements is almost destitute 

 of sculpture or granulation. 



Affinities and differences. — Pseudodiadema Mooreii resembles Pseudodiadema depres- 

 sant, Agassiz, in the depression of its upper surface and the flatness of its base, likewise 

 in having the tubercles of both areas nearly of a uniform size below ; but it is 

 readily distinguished from Pseudodiadema depressum by the number and greater develop- 



