from the Lias and Oolites. 27 



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 secondary 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 sir atigraphical .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 existing among the Echinoderms of the 

 European Oolitic fauna. 



Diadema Moorei, Wright. PL II. 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 /(jths of an inch, transverse diameter gths of an inch. 



Description. There is much difficulty in distinguishing some 

 of the smaller Diadeinas 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. 



