ON THE ECIIINOIDEA. 455 



DIADEMAD.E. 

 G^m^5— HETEROCIDARIS, Cotteau, 18 GO.* 



Test large, circular, depressed, inflated at the sides, sub-convex above, almost flat 

 below ; the inter-ambulacral areas very wide, and provided with from six to eight rows of 

 large, nearly equal-sized, perforated tubercles, raised on prominent bosses, with crenulated 

 summits ; the areolae are narrow, and their circumference surrounded by a circle of 

 small, equidistant granules, a few only of which are distributed on the intermediate surface 

 of the plates. 



The ambulacra! areas are straight, very narrow, and slightly flexuous above ; they are 

 furnished with two rows of small, distinct, perforated tubercles, uniform in size, and 

 raised on small bosses, which are placed in regular rows on the margin of the area ; three 

 tubercles occupying the depth of each inter-ambulacral plate. 



The poriferous zones are narrow ; the pores are small, simple, non-conjugate, and 

 superimposed, having a shght disposition to a trigeminal arrangement near the mouth. 



The mouth-opening is large and pentagonal, about one third the diameter of the test ; 

 from the narrowness of the ambulacra, the lobes of this portion of the peristome are 

 much smaller than those of the inter-ambulacra. 



The spines are long and cylindrical; their surface is covered with fine, longitudinal 

 lines, having small, indistinct tubercles interspersed amongst them. 



I refer this genus to the family Diademad^, as I have defined it.f The size of the 

 test, the narrowness of the ambulacra, the width of the inter-ambulacra, and the numerous 

 rows of primary tubercles thereon, indicate that Heterocidaris has certain affinities with 

 Astropyga, although it possesses many characters by which it is readily distinguished from 

 that genus. Heterocidaris resembles some of the large forms of Heviipedina, as 

 H. MarcJiamensis, Wr., but the deep crenulations on all the bosses shows it to be distinct 

 from that form. M. Cotteau observes,! that the genus Heterocidaris, notwithstanding its 

 resemblance to the Diademad^, is separated from that family by a character of the 

 first order, namely, the structure of the peristome, which is pentagonal, and furnished 

 with ambulacral lips much more narrow than those which correspond to the inter- 

 ambulacra, whilst in the Dtademad^ the peristome is always decagonal, and notched 

 by ten incisions more or less deep. He therefore places this genus in the family 

 CiDARin^ of Desor, which forms, according to that author, however, a much larger 

 group than the family Cidarid^ of this Monograph. § 



* Extrait du 'Bulletin de la Societe Geologique de France,' 2me serie, torn, xvii, p. 378, pi. iv. 

 f See p. 18 of this Monograph. 



X Extrait du ' Bulletin de la Societe Geologique de France,' 2me serie, torn, xvii, p. 380, pi. iv. 

 § See p. 18. 



