from the Lias and Oolites. 171 



The mouth is decagonal and of moderate size : the arches over 

 the bases of the ambulacra! areas are about one-third greater in 

 span than those across the interambulacra. The disc is absent 

 in all the specimens^ four in number, we have seen. 



Affinities and differences. — This species resembles Cidarites 

 [Diadema) mamillanuSy Roemer, in the prominence of the tuber- 

 cles and depression of the test; but Roemer's figure* is so indi- 

 stinct and devoid of details, that it is impossible to institute a 

 strict comparison between our Urchin and the one figured by 

 him. The difference between D. Davidsoni and D. subangulare' 

 is so marked that it is impossible to mistake them — the penta- 

 gonal outline, large tubercles, wide granulated space between 

 the primary tubercles of both areas, with the pores arranged in 

 double files on the upper surface of the test, form a group of 

 characters which distinguish D. subangulare from our Urchin. 

 From D. pseudodiadema it is distinguished by the tubercles in 

 D. Davidsoni of the ambulacral equalling in size those of the 

 interambulacral areas, whilst in that species they are unequal. 

 The rudimentary condition of the secondaiy tubercles in our 

 species forms a striking contrast to the size they attain in 

 D. pseudodiadema. The naked condition of the central parts of 

 the interambulacral areas connects it with D. subnudum, Ag., of 

 the Chalk, and the neatness of its outline allies it with other cre- 

 taceous forms. 



Locality and stratigraphical range. — We have collected this 

 species in the clays of the Coral rag near Calne, Wilts ; it is a 

 very rare Urchin, as we only know four specimens of it. 



We dedicate this species to our friend Thomas Davidson, Esq., 

 whose learned monographs on the Brachiopoda have earned him 

 the gratitude of all palaeontologists. We take this opportunity 

 likewise of recording our deep obligations to Mr. Davidson for 

 many friendly acts of assistance given during the preparation of 

 these memoirs, by which we have been enabled to compare a con- 

 siderable number of foreign Echinidse with those of our own 

 island, and thereby have been enabled to obtain a better know- 

 ledge of the affinities e 

 European Oolitic fauna. 



ledge of the affinities existing among the Echinoderms of the 



Diadema Mooreii, Wright. PI. XII. fig. 3, a-d. 



Test circular, depressed ; ambulacral tubercles smaller than those 

 of the interambulacral areas ; plates of the test covered with a 

 small wide-set prominent granulation ; mouth large and deca- 

 gonal ; anal opening large ; apical disc of moderate size. 



Height /oths of an inch, transverse diameter |§ths of an inch. 



* Die Vcrateinerungcu dcs Norddcutschen Oolitben Gcbirgcs, pi. 2. 



fig. 1. 



