312 COLLYRITES 



The mouth is more or less sub-central, and lodged in a concavity formed by the 

 prominent nodulated inter-ambulacra. It is obscurely decagonal, and appears as if round ; 

 in some specimens it is much nearer the anterior border than in others. The mouth and 

 anus are nearly of the same size, and about one eighth of the length of the shell ; no traces 

 of jaws have yet been found, nor has the structure of the peristome been sufficiently 

 made out. 



Affinities and differences. — Many specimens of this urchin agree with M. Desor's 

 figures of Disaster Eudesii, Agass., whilst others have the depressed dorsal surface, 

 and angular outline of D. ringens, Agass. As I have many series of intermediate forms 

 connecting these two extremes, I have referred them all to one species. On this 

 subject, M. Cotteau observes that he collected, with M. Moreau, from the " Oolite 

 ferrugineuse " of Tour du Pre, a suite of specimens of B. ringens; these presented 

 various degrees of tumidity and more or less circularity of outline ; among them were 

 all the gradations conducting to B. Eudesii, Agass., from this he concluded that the 

 urchin figured in his excellent work, and which may be taken as a fair representation 

 of many of our specimens, is a small and more elongated variety of D. ringens, Agass.* 

 This conclusion. Professor Forbesf admits, agrees with the experience of the collectors 

 of the Geological Survey. 



Professor M'Coy, in his memoir 'On some new Mesozoic Radiata,'J enumerates 

 D. Eudesii as a British species from the Inferior Oolite of Dundry and Bridport, and has 

 described another form under the name D. sub-ringens. As he has favoured me with 

 a sketch of this urchin, I can state with certainty that it is only a large individual of 

 D. ringens. The characters which Prof. M'Coy regarded as specific, namely, the " greater 

 gibbosity, and less prominence of the ridges on the under side," and, the " dispro- 

 portionate narrowness of the three anterior ambulacra, as in D. ringens,'' vary almost in 

 every one of the many individuals I have collected, I therefore do not hesitate to include 

 D. sub-ringens among the synonyms of Collyrites ringens, Agass,, which is distinguished 

 from its congeners by the convexity of the basal portions of the inter-ambulacra, and 

 especially by that of its single inter- ambulacrum. 



Locality and StratigrapJdcal position. — All my specimens have been collected from 

 the marly vein which traverses the upper ragstones of the Inferior Oolite of Dorsetshire, 

 in the zone of Ammonites ParJdnsoni, Sow. From this stratum I have collected it 

 between Sherborne and Yeovil, at Burton Bradstock, Walditch Hill, and Chideock Hill, 

 near Bridport. It is generally associated with Collyrites ovalis, Leske, Holectypus 

 hemispharicus, Agass., Clypeus alius, M'Coy, and StomecJdnus bigranularis, Lamck. 



* ' Etudes des Ecliiuides Fossiles,' p. 48, 



t 'Memoirs of the Geological Survey,' decade iii, pi. 9. 



X 'Annals of Natural History,' 2d series, vol. ii, p. 420. 



