MARCOU ON THE NEOCOMIAN AND THE WEALDEN HOCKS. 7 
the Ilemicidaris Purheckensis is regarded as a rare fossil in Purbeck, 
and also in Burgundy. 
During my explorations of the Jui'a in 1844-47, I met with 
fragments of a Hemicidaris in the Salins Limestone several times ; 
and when Forbes published his Hemicidaris Purheckensis, I perceived 
at once the possibility of an identity with the Jura sea-urchin ; 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. Etallou 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 Purheckensis 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 
hiilla, Sow., and Ostrea distorta, Sow,, but nothing positive as yet. 
Eelying 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 Purheckensis 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 alaia, Sow., known in the Ash- 
bumham beds of Pounceford, near Burwash, Sussex. 
