STOMECHINUS. 203 



Ge»*«— STOMECHINUS,* Desor. 1854. 



M. Desor has separated from the genus Echinus all those urchins which have a large 

 mouth opening, with the peristome nearly pentagonal, through the great development of the 

 ambulacral, and the rudimentary size of the inter-ambulacral lobes ; and which have 

 two wide, deep notches at each of the five angles of the pentagon. 



This new genus or section of the Echinid^e is composed of urchins of moderate size, 

 having a hemispherical, globular, or conoidal test, which is sometimes more or less depressed 

 in different species. 



The ambulacral areas are about one third the width of the inter-ambulacral, having 

 two marginal rows of small tubercles, from twenty to thirty in each row, and sometimes 

 two additional internal rows, in general smaller than those of the other segments. 



The inter-ambulacral areas have two principal rows^ of tubercles in the centre of the 

 plates, and several secondary rows, at their sides, often as large as the principal ones ; the 

 miliary zone is sometimes broad and granular, or narrow, depressed, and more or less 

 naked. 



The apical disc is small, the genital plates are nearly equal-sized, and the spongy 

 madreporiform body is large and prominent on the surface of the right anterior genital 

 plate. 



The mouth opening is large, the peristome is deeply notched at the base of the inter- 

 ambulacral areas ; the pairs of notches approach each other so close, that they leave only a 

 small triangular lobe between them ; the length of the ambulacral lobes is such, that it 

 produces a pentagonal-shaped mouth ; the notches have the margin reflected at each of 

 the angles. 



The poriferous zones are moderately wide, and the pores are arranged in triple oblique 

 pairs, as in the genus Echinus. 



The spines are small, short, stout, and blunt pointed; their surface is covered with 

 well-marked longitudinal lines. 



This genus is extinct, and appears to be limited to the Oolitic formations. The species 

 are found in the Inferior Oolite, Great Oolite, Cornbrash, Coral Rag, Kimmeridge Clav, 

 and Portland Oolite ; they, however, attained their greatest development in the lower 

 division of the Oolites. 



From nrouii, mouth. 



27 



