72 Dr. T. Wright on Fossil Echinoderms 
dise ; base slightly concave; mouth central; anus marginal ; 
ambulacral sulci bifid and branched. 
Dimensions.— Antero-posterior diameter 2,8, inches, trans- 
verse diameter 35%, inches, height 2/>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. Faujasii, 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. Faujasit than in S. subrotunda; the test is more 
produced posteriorly, the margin is more sinuous, the ambulacral 
