ECHINODERMATA. 37 



the areolas are circular and deeply sunk (fig. 2 c) ; around the margin of each there is a 

 complete elevated scrobicular circle of small close-set granules ; the mammary boss rises 

 abruptly from the centre, and its summit is sharply crenulated ; the tubercle is propor- 

 tionately small, and deeply perforated with a small hole ; the elevation of the scrobicular 

 circle gives an excavated air to the areolas of this species, and produces, at the same time, 

 a marked zigzag depression down the centre of the areas ; the miliary zone is wide, and 

 filled with six rows of fine granules, rather smaller in size than those forming the 

 scrobicular circles. 



The mouth opening is small, being less than one half the diameter of the test. In a 

 small specimen measuring ££ths of an inch in diameter, that of the peristome was a little 

 more than ^ths of an inch. This opening lies in a slight depression of the under surface ; 

 the peristome has a pentagonal form ; and the ambulacral areas retain their full diameter 

 to the margin of the same. 



The apical disc is absent in all the specimens ; but the dimensions of the aperture 

 occupied by these plates is very considerable, being more than half the diameter of the 

 test. In a small individual before us, measuring |§ths of an inch in diameter, the diameter 

 of the opening for the apical disc measures ^ths of an inch. In this small specimen, in 

 fine preservation, which we collected in Dorsetshire, the margin of the discal opening is 

 entire, so that we can state with certainty its comparative measurement, with that of the 

 equatorial diameter of the test itself. It is the only individual that has afforded this 

 information, all the others being more or less crushed, broken, or otherwise distorted. 



Affinities and differences. — Cidaris Bouchardii has been mistaken for, and registered 

 as Cidaris elegans, Goldf., but, although it has some affinities with that species, its dif- 

 ferences are numerous and important. The ambulacral areas in both species have two 

 rows of marginal granules, and the poriferous zones in both are of about the same width 

 and depth. In the inter- ambulacral areas the differences are conspicuous : the number of 

 tubercles is greater in Cidaris Bouchardii than in Cidaris elegans ; and the sunk areolas, 

 with their elevated scrobicular circle, form another important distinction between our 

 urchin and the German form. The tubercles, moreover, are larger and much more pro- 

 minent in Cidaris elegans : it likewise belongs to a higher stratigraphical zone, the speci- 

 mens before us having been found in the Coral Rag of Streitberg and Sigmaringen. 

 Another fact, of a negative character it is true, but not the less valuable in palaeontological 

 investigations, is this, that we have never seen spines at all approaching the curious and 

 singular character exhibited by those of Cidaris elegans in the Inferior Oolitic rock, from 

 whence our urchins have been collected. 



Cidaris Bouchardii resembles Cidaris coronata, Goldf., in many points. It has the 

 depressed form, the flexuous ambulacra, and the prominent scrobicular circle of this German 

 species, but is distinguished from it by the following characters : The equatorial portion of 

 the ambulacral areas in Cidaris coronata has four rows of granules, which diminish to two 



