72 Dr. T. Wright on Fossil Echinoderms 



disc ; base slightly concave ; mouth central ; anus marginal ; 

 ambulacra! sulci bifid and branched. 



Dimensions. Antero-posterior diameter 2 T 8 ^ inches, trans- 

 verse diameter 3 T ^ inches, height /^ths of an inch. 



Description. The test of this delicate Urchin is extremely 

 flat ; it has an irregular suborbicular discoidal form, with a thin 

 sinuous margin; the dorsal surface is regularly and gently 

 convex. The ambulacral areas are more than half the length of 

 the diameter of the test ; they are of an oblong form, lanceolate 

 above, and more obtuse below. The pores in the avenues are 

 widely apart ; those in the inner series are round, whilst those 

 in the external series terminate in slits that extend about half 

 way across the interporiferous spaces. The apical disc is large, 

 and the elements thereof are intimately soldered together. The 

 madreporiform tubercle occupies the centre, and the four genital 

 pores are pierced at unequal distances around it ; the anterior 

 pair are smaller and closer together than the posterior pair ; the 

 five ocular pores are very small. The margin of the disc is very 

 thin, and has a sinuous outline ; five of the curves thereof cor- 

 respond to the ambulacral areas, and those appertaining to the 

 postero-lateral areas are the deepest and best defined ; a small 

 notch indicates the site of the anal opening. The ventral surface 

 is slightly concave. The mouth, about two lines in diameter, is 

 central and subpentagonal ; from the angles thereof, five ambu- 

 lacral sulci radiate outwards, which soon become bifid, each trunk 

 becoming dichotomously branched in old individuals. The anal 

 opening is round, about half the diameter of the mouth, and is 

 situate near the posterior border. The tubercles are small, and 

 closely placed together ; they are nearly of a uniform size on the 

 dorsal surface. 



Affinities and differences. S. subrotunda so closely resembles 

 S. striatula, S. Fat/jasii, and S. producta, that it requires an at- 

 tentive study to discover the differences between them. As we 

 possess single specimens only of these forms, determined and 

 presented to us by M. Michelin of Paris, we are certain of their 

 identity with the types they represent. The test is narrower 

 before, and the ambulacral areas are much smaller in S. striatula 

 than in S. subrotunda; the ambulacral areas are wider, their 

 bases and apices are more obtuse, their sides flatter, and their ter- 

 minations are more truncated, andthe anus further from the border 

 in S. producta than in S. subrotunda ; the test is more convex on 

 the dorsal surface, the apical disc is wider, the margin is thicker, 

 the base flatter, and the anal aperture much further from the 

 border in S. Faujasii than in S. subrotunda ; the test is more 

 produced posteriorly, the margin is more sinuous, the ambulacral 



