172 Dr. T. ^yl•ight o)i neir Species of Echinodcrninta. 



Description. — There is inucli difficulty in distinguishing some 

 of the smaller Diademas from each other, inasmuch as the young 

 condition of many of the larger species so closely resembles the 

 adult state of others, that it is only after one obtains a number 

 of individuals of different species in their various phases of 

 growth, that the naturalist feels himself upon sure ground when 

 he endeavours to distinguish the affinities and differences that 

 exist among them. 



After a diligent search for Urchins in the Lias of Glouces- 

 tershire, we have succeeded in collecting only a very few examples 

 of this group from these rocks. In addition to those found here, 

 our friend Mr. Moore of llminster kindly presented us with a few 

 specimens which he collected from the Upper Lias near lluiinster, 

 and from these collective materials the species under consider- 

 ation was discovered. Diadema 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. The ambu- 

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

 of the interarabulacral ; their margins are occupied by two rows 

 of tubercles about eight in each row, which, at the base and up to 

 the equator, are nearly as large as those of the interambulacra ; 

 but from that region to the apex of the area they rapidly dimi- 

 nish 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. The interambulacral areas are 

 wide and well developed, and have two rows of tubercles, from 

 8-9 in each row, which occiipy the centre of the plates ; the 

 areolas of the tubercles on the upper surface are surrounded with 

 a circle of granules which sepai'ates them from each other, but 

 those of the base are confluent above and below. The inter- 

 tubercular surface at the base of the test has a number of gra- 

 nules scattered over it, whilst on the upper surface, the plates are 

 destitute of any other ornament beyond the faint circles that 

 surround the tubercles. The pedal pores are arranged in pairs 

 in a single file ; the avenues are, however, rather flexuous be- 

 low ; the basal tubercles of both areas are nearly alike in size, 

 but on the dorsal sui'face those of the ambulacra dwindle into 

 lar"-e granules, whilst those of the interambulacra maintain their 

 size up to the last pair, which are small near the margin of the 

 disc. The mouth-opening is large, and its margin is divided 

 into ten nearly equal-sized lobes. The apical disc is partly pre- 

 served in the specimen here figured ; it consists of five large ovarial 

 plates of a heptagonal form ; two of the sides unite with the in- 

 terambulacral plates, two with the ocular, two with the adjoin- 

 in" ovarials, and the single surface contributes to form the 

 boundary of the anal opening, which is of moderate size ; the 



