FROM THE INFERIOR OOLITE. 113 



ambulacral areas are nearly of a uniform breadth throughout ; there are about ten pairs of 

 tubercles in each area. In consequence of these segments of the test being double the 

 width of the ambulacra, the tubercles stand more apart (fig. 2 d). The tubercles of both 

 areas are nearly uniform in size j those of the ambulacra are the smallest ; they have a 

 smooth base, with a finely crenulated summit, and are perforated (fig. 2 h) ; there are no 

 secondary tubercles, but the miliary zones are covered with small granulations, which are 

 closely set together on the surface of the plates ; three or four of these at the base 

 are perforated (fig. 2y). The mammillary eminences of both areas are surrounded by 

 smooth areolas, which are nearly all confluent (fig. 2 d). The ambulacral areas become 

 rapidly contracted towards the vertex (fig. 2 a), whilst the inter-ambulacral maintain their 

 breadth, so that the space between the rows of primary tubercles is very uniform in 

 width throughout (fig. 2 a, e). The miliary zones, with the exception of the internal 

 border of the four superior inter-ambulacral plates, are covered with small, close-set 

 granulations of different sizes (fig. 2 d), which form semicircles around the areolas, and 

 zigzag lines down the centre of the areas. The pores consist of from thirty-six to forty 

 pairs in each avenue, super-imposed in a single file ; about four pairs of pores are opposite 

 each large inter-ambulacral plate (fig. 2 d) ; in the wide space of the avenues around 

 the mouth, they form rows of triple oblique pairs (fig. 2 </). The mouth is large and 

 decagonal (fig. 2 b) ; the notches of the peristome divide the opening into ten nearly equal- 

 sized lobes, the borders of the notches are reflexed at the angles; the apical disc is unknown ; 

 the spines are small, subulate, and delicately striated longitudinally (fig. 2 e,f, i). 



Affinities and differences. — This urchin resembles Pseudodiadema aquale, Agassiz, but 

 differs from it in the absence of secondary tubercles in the inter-ambulacral areas ; by its 

 pentagonal form it resembles Pseudodiadema versipora Phillips, but is distinguished from 

 that species in having the pores arranged in a single file ; whereas in Pseudodiadema 

 versipora, from the equator to the apical disc, the pores fall into double rows. The 

 tubercles are likewise smaller, and more deeply perforated ; it belongs, moreover, to a lower 

 zone of the Oolitic group, Pseudodiadema versipora being a characteristic urchin of the 

 Coral Rag of England and the "Terrains a Chailles " of Switzerland.* Like Pseudo- 

 diadema versipora, Pseudodiadema depressum possesses a pentagonal form, a peculiarity 

 depending on the prominence of the ambulacral areas, and common to several species of 

 this genus. 



Locality and Stratigraphical position. — This urchin is common in the lower ferruginous 

 beds of the Inferior Oolite, the Pea Grit, at 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 Inferior Oolite specimens are in general much crushed ; 



* Agassiz, ' Echinodermes Fossiles de la Suisse.' 



15 



