MARCOU ON THE NEOCOMIAN AND THE WEALDEN ROCKS. 7 



the Hemicidaris PurhecJcensis is regarded as a rare fossil in Purbeck, 

 and also in Burgundy. 



During my explorations of the Jura in 1844-47, I met with 

 fragments of a Hemicidaris in the Salins Limestone several times ; 

 and when Forbes published his Hemicidaris Purleckensis, I perceived 

 at once the possibility of an identity with the Jura sea-urchin j and on 

 making a rapid excursion to Portland in 1852, I saw immediately 

 that the strata called in the Jura Portlandian were not equivalent to 

 the Portland-stone of England, but a little younger. Having learned 

 that a well-preserved Hemicidaris had lately been found by M. Perron, 

 of Gray,, in the Salins Limestone near that town, I wrote to call his 

 attention to the subject ; and the result of researches made by him 

 and M. Etallon is, that the Hemicidaris of Gray is identical in all 

 respects with the Hemicidaris Purheckensis. MM. Perron and Etallon 

 say that their specimens do not indicate any variations from the true 

 Hemicidaris Purheckensis of Professor Edward Forbes ; and this beau- 

 tiful fossil is quite common even with the spines adherent to the shell. 



The exact position of the Hemicidaris PurhecJcensis at Gray is about 

 thirty feet from the base of the Salins Limestone. There are also some 

 indications of the existence in the Salins Limestone of the Exogyra 

 hulla, Sow., and Ostrea distorta, Sow., but nothing positive as yet. 



Relying only on the Hemicidaris Purheckensis, it is, however, quite 

 probable that the Salins Limestones are the marine deposits coeval 

 with the Purbeck beds ; especially if we consider that in England a 

 change of some note takes place in the distribution of deposits ; for 

 the Hastings Sands and Weald Clay range through a very different 

 part of the country from the Purbeck strata. The discovery of the 

 Hemicidaris Pu7'heckensis in the last division of the Jurassic rocks in 

 the Jura mountains shows the soundness of Forbes' view when he 

 replaced the Purbeck beds in the English Oolites. 



The Villars Marls contain a fluvio-marine fauna, which will aid us 

 in the endeavour to find the equivalents in the two countries, and the 

 more as we now know that Professor Lory, of Grenoble, has found in 

 Dauphine the marine deposit coeval with them. Until now, only one 

 species truly identical with an English fossil has been discovered in 

 the Villars Marls : it is the Corhula alata, Sow.,, known in the Ash- 

 burnham beds of Pounceford, near Burwash, Sussex. 



