FROM THE CORAL RAG. 217 



Affinities and differences. — M. Desor, by mistake, has placed Stomechinus gyratus as a 

 synonym of Stomechinus germinans, and described the true Stomechinus germinans as a 

 new species under the name Stomechinus sub-conoideus, a confusion which has doubtless 

 arisen from the stratigraphical error already pointed out in the history of St. germinans. 

 The following characters will show how entirely distinct St. gyratus is from the three 

 closely allied forms of the Inferior Oolite. It differs from St. germinans in having the 

 naked median sulcus wider and deeper, and extending further down the sides ; in having 

 four rows of equal-sized tubercles in the ambulacra, whilst St. germinans has only two. 

 The secondary tubercles of the inter-ambulacral areas are as large as the principal row 

 and there is no miliary zone in this species ; whereas, in Stomechinus germinans^ 

 intermedins, and bigranularis, the miliary zone is a very wide granular space. The 

 tubercles are likewise larger and nearly of the same size in both areas. I have been able to 

 examine Echinus petallatus, M'Coy, through the extreme kindness of Professor Sedgwick, 

 and compared it with the urchin I have figured ; I can, therefore, state that it is an un- 

 questionable specimen of Echinus gyratus, Agass., and not in any way a distinct form, as 

 Professor M'Coy supposed. 



Locality and Stratigraphical position. — Stomechinus gyratus is rarely found in the Clay 

 beds of the Coral Rag, near Calne, Wilts ; I am not aware that it has been collected in 

 any other locality in England. Its foreign locality is likewise limited to the " terrain a 

 Chailles" or Corallian stage of Besancon. I have a specimen from the Coral Rag of the 

 department of Haute Saone, France, kindly sent me by M. Michelin. 



History. — First described by M. Agassiz in his ( Echinoderrnes Fossiles de la Suisse,' 

 where it is beautifully and accurately figured. The English form was afterwards described 

 by M'Coy as Echinus petallatus. It was first described in detail in my ' Memoir on the 

 Cidaridse of the Oolites,' and its distinction from our other Oolitic Echinidae was therein 

 indicated. It is probable, that Parkinson's figure, in vol. 3 of his ' Organic Remains, 

 described as an ' Echinite from France/ refers to this species. 



Stomechinus nudus, Wright, nov. sp. PI. XV, fig. 2 a, b, c, d, e. 



Test circular, conoidal ; ambulacra flat, with two regular marginal rows of tubercles, 

 twenty-four in each row, and two inner rows which occupy the middle half of the area ; 

 poriferous zones wide ; pores oblique ; two small granules between each trigeminal rank • 

 inter-ambulacra with eighteen plates in each column ; equatorial plates with four or five 



