ECHINODERMATA. 57 



Rag of Nattheira, Besancon, Chatel-Censoir, and Druyes (Yonne) ; the only exception to 

 this being a spine (PL I, fig. 5) found by me in the Pea Grit, Inferior Oolite, of Crickley 

 Hill, and placed by M. Desor in this genus. 



The inter-ambulacral plate is large, as deep as it is wide, and must have formed 

 part of a very large test ; the areola is rather more than two thirds of the width of the 

 plate, it is circular, and is placed so near the poriferous zone that its scrobicular circle 

 abuts against the pores. The areola is smooth, superficial, and not excavated ; the boss 

 forms an inconsiderable prominence in the centre, and has a broad summit, whioh was 

 crenulated, although the remains of the markings are now nearly effaced. The tubercle is 

 small in proportion to the size of the plate ; the miliary zone must have been very wide, 

 and filled with four or six rows of large granules, which stand at distinct intervals apart. 

 The scrobicular circle is complete ; the granules forming it are set upon an elevated rim of 

 the areola, but they are not larger than the other granules covering the surface of the 

 plates. 



The ambulacral areas are very narrow and flexuous, having two rows of small granules 

 on their margins ; the poriferous zones follow the windings of the area, which are more 

 flexuous in this species than in the German Coral Rag forms, in consequence of the 

 proximity of the scrobicular circle to the zones themselves ; in fact, the absence of flexures 

 in Diplocidaris gigantea led M. Desor to state that the ambulacra were straight in this 

 genus ; whereas they are both straight and flexed, in proportion as the tubercles occupy 

 the middle or the zonal sides of the plates. 



The poriferous zones are narrow ; the pores are round and contiguous, separated by 

 septa, about as thick as the diameter of the holes ; on the surface of the septa the test is 

 elevated, and forms small, blunt granules ; the pores are not, strictly speaking, bigeminal, 

 but form an irregular series, every pair being more or less oblique to the pair above them 

 and below them ; they may be described as forming double oblique pairs, thus, ':': so that 



every third pair of holes stand vertically above a third pair below them, a form of arrange- 

 ment very different from that prevailing in the genus Cidaris, in which the pores are 

 strictly unigeminal throughout the entire zones. 



Affinities and differences. — Diplocidaris Desori differs from Diplocidaris gigantea in 

 having deeper rhomboidal plates ; and the areolas and tubercles are placed closer to the 

 poriferous zones ; the ambulacral areas are likewise narrower ; the pores form double 

 oblique pairs in the zones, instead of making two distinct series as in Diplocidaris gigantea. 



Locality and Stratigraphical position. — This fragment was discovered near Yeovil, 

 in a rock supposed to be Inferior Oolite, but which probably may be Upper Lias. It 

 is impossible to say anything upon this point, unless the Ammonites with which it was 

 associated were before me, inasmuch as the stratigraphical line between the Upper Lias 



8 



