FROM THE GREAT OOLITE. 119 



two rows of primary tubercles, eight to nine tubercles in each row ; those at the equator 

 are nearly as large as the tubercles in the inter-ambulacra in the same region of the test 

 (fig. 5 d) ; the summits of the bosses are sharply crenulated, and the tubercles are small 

 and perforated ; a zigzag row of small granules extends down the middle of the area 

 (fig. 5 e), and small transverse branches shoot out from the sides, and extend to the 

 poriferous zones, separating the bosses from each other, and forming imperfect areolar 

 circles around them. 



The poriferous zones are narrow, and the pores are unigeminal throughout (fig. 5 e) ; 

 the zones approach each other very closely in the upper part of the test (fig. 5 b), which 

 gives to the inter-ambulacra the uniform width they have (fig. 5 b, d). 



The inter-ambulacral areas are a very little wider at the equator than the ambulacral 

 (fig. 5 c) ; from the equator upwards they maintain a very uniform width (fig. 5 b) ; 

 there are nine tubercles in each of the two rows, which are set on small bosses, with 

 sharply crenulated summits; the tubercles are prominent and perforated (fig. 5/); the areolas 

 are confluent above and below (fig. 5 b) ; between the tubercles and the poriferous zones a 

 row of unequal-sized granules extends (fig. 5 e), which forms one side of the scrobicular 

 circle ; the miliary zone is narrow, consisting, from the peristome to the equator, of two 

 rows of granules, from the equator upwards a few more granules are interspersed with 

 them, these form the other portion of the scrobicular circle (fig. 5 e) ; the upper part of 

 the zone is naked in the region of the central suture (fig. 5 b). 



The mouth opening is large, the peristome is unequally decagonal (fig. 5 c), the ambu- 

 lacral lobes being the largest. The base of the test (fig. 5 c) shows how uniform the 

 structure of this Pseudodiadema is ; and the uniformity in the size of the tubercles in this 

 region gives value to its specific name, homostiyma. 



The discal opening is small, and nearly circular ; but the plates of the disc are absent. 



Affinities and differences. — This species is distinguished from the smaller forms of 

 Pseudodiadema depressum in being nearly circular, in having wider ambulacra, a smaller 

 discal opening, and more equal-sized tubercles. It has many affinities with Pseudodiadema 

 Bailyi (PI. VI, fig. 1); but is distinguished from it in having proportionately wider 

 ambulacra, the primary tubercles likewise are of a more uniform size throughout the area, 

 those of Pseudodiadema Bailyi diminishing more rapidly in the upper part of this region. 



Locality and Stratiyraphical position. — I have collected this species in the Bradford 

 Clay, near the Tetbury-road Station of the Great Western Railway, and Mr. William Buy 

 has collected it in the Cornbrash near Sutton-Benger, Wilts. It is rather a rare form in 

 these beds. My excellent friend, the Rev. W. Griesbach, has kindly sent me a specimen 

 which he found in the marly beds of the Great Oolite at Wollaston, Northamptonshire, 

 associated with several Great Oolite and Cornbrash species of Echinodermata. 



