FROM THE INFERIOR OOLITE. 211 



restrict the name bigranularis to the Oolitic species, so well figured by M. Wright under 

 the name serialis, and afterwards by M. Forbes."* I am the more disposed to accept 

 this determination, as I have a beautiful specimen of this species from the Ferrugenous 

 or Inferior Oolite of Croisilles, Calvados, kindly sent me by M. Michelin as Echinus 

 bigranularis, Agassiz, and determined by him, which exactly corresponds with our English 

 specimens. 



I have been fortunate in obtaining a fine series of this urchin, which has enabled me 

 to study the variations it presents at different periods of growth ; with the exception 

 of some forms being more globular, others more pentagonal, all its essential characters are 

 extremely persistent in my specimens. 



The test is sometimes globular, but in general hemisperical and depressed ; the cir- 

 cumference is occasionally round, but oftener sub-pentagonal, and varies in size from one 

 to two inches in diameter. The specimen figured is an extremely pentagonal variety, and 

 was selected because the tubercles and sculpture of the test were finely preserved 

 (fig. 3 a, c). 



The ambulacral areas are narrow, and nearly of a uniform width throughout. ; they 

 have two rows of small tubercles, placed wide apart on the margins, of which there are 

 twenty-six in each row (fig. a, c) ; every alternate plate in the column carries a tubercle, 

 which alternates with a plate covered with two rows of small granules (fig. 3 d) ; at the 

 base the tubercles are larger (fig. 3 b), and in the middle two rows of small granules 

 separate the marginal tubercles (fig. 3 d). 



The poriferous zones are narrower than in St. germinans and St. intermedins ; the 

 trigeminal ranks incline at angles of from 55° to 60°; they become more sub-parallel on 

 the upper part of the zones, and towards the base ; there is one small tubercle at the angle 

 between each file (fig. 3 d) of pores, and there are five trigeminal ranks opposite two 

 tubercular plates. 



The inter-ambulacral areas are three and a half times as wide as the ambulacral 

 (fig. 3, a, c) ; the plates are broader than in the preceding species ; and consequently there are 

 only from twenty to twenty-two plates in a column. There are two rows of primary 

 tubercles, which are placed nearer the poriferous rows than the centro-suture (fig. 3 c) ; 

 they are a little larger than the ambulacral tubercles, and nearly of the same size from the 

 circumference upwards. A row of smaller secondary tubercles, about twelve in number, 

 occupies their zonal side, and extends half way up the test (fig. 3 c). The tubercles are 

 raised on bosses (fig. 3 d), surrounded by narrow, ring-like areolas, a series of small 

 granules encircle the base, and the remaining surface of the plates is dotted over with 

 numerous small equal-sized granules (fig. 3d). There is a very slight median depression, 

 from the upper part of which the small granules are absent (fig. 3 a). At the base of the 

 area six larger tubercles are disposed within the primary rows (fig. 3 b). 



* 'Synopsis des Echinides Fossiles,' p. 125. 



28 



