74 PEor. p. M. Duncan's revision op the 



ambulacral plates near the ambitus, and fig. 4 above the ambitus ; 

 the difBerence is remarkable. The adoral pair of pores (fig. 4) 

 is nearer the ambulacral median line than the others, as in 

 HcMnothrix. 



Genus Stomechinus, Desor, 1858, Synopsis, p. 124. Duncan, 

 1885, Quart. Journ. Geol. Soc. vol. xli. p. 435. De Loriol, 

 1887, Faune Cret. du Port., EcJi. fasc. i. p. 65, pi. x. 



Test moderate and large, circular in tumid ambital outline, 

 subconical or subhemispherical dorsally, more or less depressed. 

 Coronal plates numerous. 



Apical system with large angular basals forming the peri- 

 proctal ring, and projecting into the interradia; radial plates 

 small- 

 Ambulacra moderately wide ; at a short distance from the 

 apical system the pairs of pores are in close, oblique, triple series; 

 three pairs to a compound plate, the adoral pair nearest the inter- 

 poriferous area. A compound plate consists of an adoral and 

 aboral low primary component, the sutures being convex towards 

 the tubercle, and a large median primary carrying the greater part 

 of the tubercle and forming the median sutural angle (diade- 

 matoid). Interporiferous areas with non-crenulate imperforate 

 primary tubercles arranged in two vertical rows, with or without 

 additional rows in the median part. 



Interradia with a primary tubercle on each coronal plate, 

 similar to, or larger than, the ambulacral tubercles, imperforate 

 and non-crenulate, thus forming two main vertical rows ; a 

 varying number of small secondary tubercles and granules on 

 either side of the primary tubercles ; median area may be bare 

 near the apex. 



Peristome large, pentagonal ; branchial grooves large, with 

 raised rims ; ambulacral margins much larger than the interradial. 

 {Spines small, short, blunt, striated longitudinally. 



Fossil. Oolite : England and Europe ; America, South. 

 Cretaceous : Europe. 



M. de Loriol's species 8. camarensis{op. cit. p. 65) is abnormal; 

 for nothing can be clearer than the " diadematoid " nature of 

 the ambulacral plates of the British types. The narrowness of 

 the ambulacra of the peristome, the small branchial incisions, and 

 the demi-plates of the ambulacra characterize M. de Loriol's 

 species, bvit do not bring it within our idea of Stomechinus. 



