FROM THE GREAT OOLITE. 213 



be a Dorsetshire fossil. I have never seen St. bigranularis in the Inferior Oolite of 

 Gloucestershire. In this county it is represented by St. intermedins. 



The only authentic foreign locality that I am acquainted with is Croisilles, Calvados, 

 where it occurs in a brown ferruginous Inferior Oolite, containing numerous large grains 

 of the hydrate of iron, the " calc. a polypiers de Croisilles," Michelin. 



History. — First figured in my ' Memoir on the Cidaridae of the Oolites' as Echinus 

 serialis, afterwards by Professor Forbes in the fifth decade, pi. 4, fig. 6, of the ' Memoirs 

 of the Geological Survey ;' where it was well described by Mr. Salter as Echinus perlatus, 

 var. Forbesii. 



B. Species from the Great Oolite — ll e Etage, Bathonien, d'Orbigny. 



Stomechinus MiCKocYr-Hus, Wright, nov. sp. PI. XV, fig. 1 a, b. 



Test circular, hemispherical, with a flat base ; ambulacral areas with four rows of 

 tubercles, diminishing to two on the upper surface ; inter-ambulacral areas with ten rows 

 of tubercles at the equator, irregularly disposed on the plates, and a median depression in 

 the line of the centro-suture ; poriferous zones wide, the pores in oblique ranks of threes, 

 and between each file two small granules regularly disposed. 



Dimensions. — Height, six tenths of an inch ; transverse diameter, nearly one inch. 



Description. — The modern generic divisions of the family Echinid^e often repose upon 

 characters which undergo many phases of development in the different species ; and thus 

 it sometimes happens, as in the urchin now under consideration, some of the species 

 approach, in their ensemble, nearer to aberrant forms of an allied genus than to the one to 

 which they are referred. 



This form certainly very much resembles a large Polgcgphus Normannus, Desor ; 

 although the state of conservation of the test, and the concealment of the base by 

 adherent rock, prevents that amount of examination so necessary for a critical diagnosis ; 

 still, however, the specimen exhibits such a group of characters, that I have placed it in 

 the genus Stomechinus for the following reasons: 1st, The size of the body; 2d, the 

 thickness of the test ; 3d, the irregular arrangement of the tubercles on the inter-ambu- 

 lacral plates ; and 4th, the absence of a median depression in the inter-ambulacra. 



The ambulacral areas (fig. 1 b) have four rows of tubercles at the equator, two marginal 

 rows placed on the extreme borders of the area, and two inner rows near the sutural line ; 

 the tubercles all alternate with each other (fig. 1 b), and the two inner rows disappear at 

 the upper surface. 



