208 STOMECHINUS. 



Stomechinus intermedius, Agassiz. PI. XIV, fig. 2 a, b, c, d. 



Echinus intermedius. Agassiz, Catalogus systematicus Ectyporum Echinodermatum, 



p. 12. 

 — granularis. Wright, Annals and Magazine of Natural History, 2d series, 

 vol. viii, p. 277. 



Test hemispherical, depressed, inflated at the sides, nearly circular at the circumference, 

 sometimes inclining to a sub-pentagonal form • ambulacral areas with two rows of small 

 tubercles on the margins, and a miliary zone between ; poriferous zones wide ; trigeminal 

 ranks very oblique ; inter-ambulacral areas with two entire primary, and four short 

 secondary rows of small tubercles ; miliary zone very wide, and covered with small 

 granules ; apical disc slightly excentral, of moderate size, genital plates fully developed ; 

 mouth opening large ; peristome pentagonal, with two small notches at each angle ; 

 inter-ambulacral areas slightly depressed at the centro-suture. 



Dimensions. — Height, one inch and one fifth ; transverse diameter, one inch and nine 

 tenths. 



Description. — Through the kindness of Professor Deslongchamps, I possess a type 

 specimen of Echinus intermedius from the Great Oolite of Ranville, and which fossil was 

 determined by M. Agassiz. This specimen has enabled me to distinguish the urchin 

 mentioned in the ' Catalogue raisonne,' and entered as var. major of Echinus bigranularis, 

 Lamarck, in that memoir. 



It is certainly very difficult to describe the nice distinctions between the first three 

 species, beautifully and truthfully drawn in PI. XIV, as they graduate into each other 

 so insensibly, that it is almost impossible to seize their distinctive characters ; so much so, 

 that at one time I considered them varieties of one species ; having, however, examined a 

 great many specimens of these Echini, I am satisfied that if varieties, they are permanent 

 ones, and as such require a separate description. Their structural characters, however, 

 are sufficiently marked to justify their separation into distinct species. 



The test of Stomechinus intermedius is hemispherical, but rather depressed above ; the 

 sides are a little inflated, the marginal angle is obtuse, and the base is flat (fig. 2 c) ; the 

 ambulacral areas are one third the width of the inter-ambulacral ; they have two marginal 

 rows of small tubercles, about thirty in each row, arranged with regularity on the margins 

 of the areas (fig. 2 a, d); these tubercles have small bosses, and narrow areolas, which are 

 encircled by minute granules ; there are two rows of granules between the tubercles and 

 the suture (fig. 2 d), which extend through the entire length of the area. 



The poriferous zones are wide, the trigeminal ranks incline at about 40° ; three small 

 tubercles are placed between each rank (fig. 2 d), and there are two ranks opposite each 



