162 Dr. T. Wright on the Cidaridse of the Oolites. 



these segments of the test being double the width of the ambu- 

 lacral, the tubercles stand more apart. The tubercles of both 

 area? are nearly uniform in size, they have a smooth base with a 

 finely crenulated summit, and are perforated ; there are no secon- 

 dary tubercles, but the intertubercular spaces are covered with 

 small granulations, which are closely set together on the surface 

 of the plates ; three or four of these at the base of the area? are 

 perforated. The mammillary eminences of both arese are sur- 

 rounded by smooth areolae, which are nearly all confluent. The 

 ambulacra! arese become rapidly contracted towards the vertex, 

 whilst the interambulacral area? maintain their breadth, so that 

 the space between the rows of primary tubercles is very uniform 

 in width throughout. The intertubercular spaces, with the ex- 

 ception of the internal border of the four superior interambu- 

 lacral plates, are covered with small close-set granulations of dif- 

 ferent sizes, which form semicircles around the areola?, and zig- 

 zag lines down the centres of the arese. The pores consist of 

 thirty-six pair in each avenue superimposed in a single file ; in 

 the wide space of the avenues around the mouth they form double 

 or triple rows. The mouth is large and decagonal ; the notches 

 are slight, and the borders are reflexed at the angles ; the apical 

 disc is unknown ; the spines are small, subulate, and delicately 

 striated longitudinally (fig. 2d). 



Affinities and differences. — This Urchin resembles D. cequale, 

 Agass., but differs from it in the absence of secondary tubercles 

 in the interambulacral arese : by its pentagonal form it resembles 

 D. subangulare, but is distinguished from that species in having 

 the pores arranged in a single file, whereas in D. subangulare, 

 from the equator to the apical disc, the pores fall into double 

 files. The tubercles are likewise smaller and more deeply per- 

 forated ; it belongs moreover to a lower zone of the Oolitic group, 

 D. subangulare being a characteristic Urchin of the Coral Rag of 

 Wilts and the " Terrain h chailles n of Switzerland and Ger- 

 many*. Like D. subangulare, D. depressum possesses a pentagonal 

 form, a peculiarity depending on the prominence of the ambu- 

 lacral area?, and common to several species of this genus. 



Locality and stratigraphical range. — This Urchin is common in 

 the lower ferruginous beds of the Inferior Oolite, the Pea-grit of 

 Crickley, Leckhampton and Dundry Hills; I have collected it 

 from the Great Oolite at Minchinhampton and from the Bradford 

 clay at Tetbury road station; the latter were extremely small. 

 The specimens are in general much crushed; the anal disc is 

 always broken, and the spines are sometimes adherent to the test. 

 It has been collected by M. D'Orbigny in the Inferior oolite of 



* Goldfuss, Petrefacta Germanise ; and Agassiz, Echinodermes Fossiles 

 de la Suisse. 



