180 Dr. T. Wright on the Cidarida of the Oolites. 



The mouth-opening is very large, and has more the appearance 

 of being pentagonal than decagonal, from the extreme shortness 

 of the arches over the termination of the interambulacral, and 

 the disproportionate span of those over the ambulacral areas. 



Affinities and differences. — This Echinus resembles some gra-^ 

 nular varieties of E. perlaius, and the concave median space in 

 the tnterambulacra of that species increases the resemblance 

 thereto ; but the following structural differences afford good 

 diagnostic characters by which the two forms may always be di- 

 stinguished from each other. In E. gyratus the naked concave 

 median space in the interambulacra is wider, and extends much 

 further down the sides than the corresponding space in E. per- 

 latus. The ambulacra] area? have four rows of tubercles in E. 

 gyratus, and only two rows in E. perlatus. The lateral tubercles 

 in the secondary ranges of the interambulacral area? are nearly 

 as large as those of the complete ranges of the same in E. gy- 

 ratus, whereas the secondary ranges of the corresponding areas 

 in E. perlatus are smaller and even sometimes rudimentary. The 

 tubercles themselves in E. gyratus are more prominent and con- 

 vex*, and are more highly polished than those of E. perlatus. The 

 same group of characters enables us to distinguish this species 

 from its other congeners. Prof. M'Coy has described an Echinus 

 from the Coralline Oolite of Wilts under the name E. petallatus, 

 which we consider to be identical with E. gyratus : we cannot 

 admit that an increased elevation of the test, or a greater extent 

 of the naked surface of the plates, are sufficient characters to 

 justify the separation of species : our specimen was obtained from 

 the same stage and locality, and as we are not acquainted with 

 any other Echinus for which it could be mistaken, we have no 

 doubt of its identity with the E. petallatus. 



Locality and stratigraphical range. — This Urchin was collected 

 from the clay beds of the Coralline Oolite of Wilts ; at present we 

 are not aware of its having been found in any other locality in 

 this country. Its foreign distribution is likewise limited to the 

 "Terrain a chailles" or Corallian stage of Besaneon. 



History. — First described by M. Agassiz in his ' Echinodermes 

 Fossiles de la Suisse/ where it is beautifully and faithfully figured ; 

 afterwards noticed by Prof. M'Coy under the name E. petallatus, 

 from the Coralline Oolite of Wilts. We are inclined to think that 

 this Urchin is the same which was figured by Parkinson in the 

 3rd vol. of his ' Organic Remains/ and described as an " Echinite 

 from France." 



