FROM THE INFERIOR OOLITE. 191 



bordered on each side with a single pair of pores ; spiniferous tubercles as in Echinus." 

 The six species included in this genus are all living. 



M. Agassiz's* definition of Arbacia differs very materially from that of the original 

 author. He confines it to "small, sub-spherical urchins, having the test covered by 

 numerous, small, smooth-based, imperforate tubercles, ranged in numerous rows on the 

 inter-ambulacral, and sometimes on the ambulacral areas. Pores disposed in simple pairs. 

 Mouth circular, without deep notches. Apical disc narrow and ring-like." All the 

 species enumerated in the ' Catalogue raisonne ' are Cretaceous and Tertiary fossils, and the 

 Arbacia of Gray are transferred to the genus Echinocidaris, Desml., so that the Arbacia 

 of Agassiz is not the Arbacia of Gray, but a new group which required revision and a 

 distinct name. To avoid future confusion, one section now forms the genus Cottaldia, 

 Desor, another the genus Magnotia, Michelin, whilst the original Arbacia of Gray are 

 placed in the Echinocidaris of Desmoulins, as that genus was first established. 



Magnotia Forbesii, Wright. PI. XIII, fig. 6 a, b, c, d, e,f. 



Arbacia Forbesii. Wright, Annals and Magazine of Natural History, 2d series, vol. viii, 



p. 278, pi. 13, fig. 4. 

 Echinus Forbesii. Morris, Catalogue of British Fossils, 2d edit., p. 79. 



— — Salter, Memoirs of the Geological Survey, Decade V, description of 



pi. 4. 

 Magnotia Forbesii. Desor, Synopsis des Echinides Fossiles, p. 115. 



Test small, hemispherical, sub-pentagonal ; ambulacral areas straight, narrow, and of 

 uniform width throughout, with four rows of small, equal-sized, close-set tubercles; 

 poriferous zones narrow, and sunk in a groove ; pores unigeminal throughout ; inter- 

 ambulacral areas wide, each divided into two lobes by a median depression, the surface of 

 the plates covered with numerous close-set, equal-sized tubercles ; base concave ; mouth- 

 opening large, situated in a depression ; apical disc small, prominent, and ring-shaped. 



Dimensions. — Height, nine twentieths of an inch ; transverse diameter, three quarters 

 of an inch. 



Description.— -The test of this beautiful sub-pentagonal little urchin is divided into 

 fifteen unequal lobes ; five of these, forming the ambulacral areas, are narrow, and ten 

 much wider, the divided inter-ambulacral areas, which have deep furrows corresponding 

 to the centro-sutural line, and dividing each area into two equal convex conical lobes; the 



* 'Catalogue raisonne des Echinides, Annales des Sciences Naturelles,' 3 me serie, tome vi, p. 3.15. 



