212 STOMECHINUS. 



The base is flat, and, as all the tubercles are of an equal size, this region of the test 

 presents a remarkable tuberculous character (fig. 3 b), when compared with its smooth 

 upper surface (fig. 3 a); the mouth opening is much smaller than in St. germinans and 

 St. intermedins ; thus, in fig. 3 b, the diameter of the test is nearly two inches, whilst the 

 width of the mouth is seven tenths of an inch ; in fact, the smallness of this opening is one 

 the most important diagnostic characters of the species, and becomes very evident when 

 we compare fig. 2 b and fig. 1 «, with fig. 3 b ; this comparative smallness of the mouth 

 is a persistent character in all the specimens I have examined, the peristome is pentagonal, 

 with two obtuse notches at each angle. 



The apical disc is of moderate size and slightly excentral, projecting backwards into the 

 single inter-ambulacrum (fig. 3 c) ; the anterior pair of genital plates are larger than the 

 posterior pair. The right plate, with the madreporiform body, is the largest, and the 

 posterior single plate the smallest (fig. 3 a, e). On the surface of each of these plates, the 

 anterior right plate excepted, there are six or seven small tubercles arranged, which 

 collectively form a moniliform ring about the margin of the vent (fig. 3 e), which is 

 extremely large in this species, and projects slightly towards the left side of the body ; the 

 ocular plates are small, and the eye-holes like oblong slits (fig. 3 e). 



Affinities and differences. — In the general form and physiognomy of the test, this species 

 resembles Stomechinus intermedins; it is distinguished from that species, however, by 

 having fewer ambulacral and inter-ambulacral tubercles, the poriferous zones narrower, 

 the trigeminal ranks of pores more upright, and the mouth opening much smaller. 

 The apical disc is larger, the anal opening much wider, with a circle of granules around 

 its margin ; on the miliary zones the granules are likewise smaller and more numerous. 

 The same group of characters serve to distinguish it still more distinctly from 

 St. germinans. 



M. Desor and M. Agassiz consider St. intermedins as a variety of St. bigranularis ; but 

 from this opinion I must beg to differ, because, although St. germinans and St. intermedins 

 may possibly be varieties of one species, still assuredly St. bigranularis is distinct from both ; 

 the smallness of the mouth opening, the structure of the apical disc, and the greater 

 breadth of the inter-ambulacral plates, in my judgment, justify the distinction. 



Locality and Stratigraphical position. — I have collected this species only from the 

 Upper Ragstones of the Inferior Oolite near Bridport, in Dorsetshire, associated with 

 Holectypns hemispharicus, Desor; Clypeus ahus, M'Coy ; Ammonites Parkinsoni, Sow.; 

 and Terebratnla spharoidalis, Sow. 



The specimen I figured in the 'Annals of Natural History' as Echinus serialis was said 

 to have been collected from Dundry Hill, but I find this was a mistake, as it turns out to 



