416 ECHINOTHRIX TURCARUM. 



Echinothrix turcarum 



Diadema turcarum ScHTNV., 1711, Tins. Iraag. 



! Echinothrix turcarum Pe.t., 1853, Monatsb. Akad. Berlin. 



/'/. //',/'• 84-48; PL III"./, s; Pi. XXIV. f. 88-36. 



This is by far the most common of the species of this genus. The test is 

 moderately thick, flattened above and below, extremely regularly arched in 

 profile j anal system small ; genital plates but slightly elongated, and ocular 

 plates separating them broadly rectangular, excepting the two adjoining 

 madreporic genital, which are more or less trapezoidal. Madreporic genital in 

 large specimens greatly exceeding the others in size, but generally in speci- 

 mens already measuring I'll" 1 " 1 -, but slightly larger or more prominent than 

 the other genital plates; genital openings large, slightly elliptical, situated 

 near outer extremity ; anal vd^i' of genital and ocular plates filled by sec- 

 ondary tubercles carrying small, slender spines similar to those of the ambu- 

 lacral system, the exterior edge only of anal system carrying small plates of 

 uniform size. The coronal plates are high, and in the largest specimens 1 

 have seen there are but three large primary t nbercles upon each plate in 

 the interambiilaeral system above the ambitus. The tubercles are well sepa- 

 rated; the secondary tubercles are small, and not more than one or two for 

 each plate, with a few miliaries filling the spaces between the primaries 

 around the scrobieular circle. The primary tubercles are large ; they are 

 uniform in size above the ambitus, verj prominent, very sharply crenulated; 

 mammary boss very distinct, and large scrobieular circle. On the actinal 

 surface they rapidly become smaller, and there are sometimes as many as 

 five primaries for each coronal plate near the ambitus in large specimens ; the 

 median space, occupied by secondaries, forms an ill-defined, irregular, verti- 

 cal row. Above the ambitus the primaries form three principal vertical rows. 

 The bare median interambiilaeral space docs not extend beyond the fourth 

 coronal plate in the largest specimens examined. Near the abactinal system 

 there are sometimes as many as four irregular vertical rows of small tuber- 

 cles in the median space between the two outer rows. The poriferous zone 

 is narrow above the ambitus, scarcely more than one third the width of the 

 median ambulacra! region. 



The actinostome has dee]) broad cuts; actinal membrane thin, supported 

 by but i'cw small elongate limestone plates in the prolongation of the am- 

 bulacra. The spines of the interambiilaeral primary tubercles are long, 

 stout, tapering, but slightly hollow, frequently equalling in length the diam- 



