6 



THE GEOLOGIST. 



I have given a rough sketch under the form of a tabular and 

 proportional view (PL I. Fig. 2) — Abstract Section of the series of 

 strata compHsed between the Portland stone and the Lower Greensandf 

 in the South-Eastern part of England — for the sake of comparison. 



The Banne Limestone and Salins Marls being the equivalents of 

 the Portland beds, and the Rhodanian presenting exactly the fauna of 

 the " Perna-beds " and crackers " of the Lower Grreensand, it appears 

 rational to conclude that the Purbeck beds, the Hastings sands, and 

 Weald clay, are fluvio-marine and terrestrial deposits coeval with 

 the marine deposits known in the Jura under the name of Salins 

 Limestone and ISTeocomian. 



A few marine fossils, or, at least, belonging to brackish -w^ater 

 animals, have lately been found common to the two series in the 

 Jura and in England, and they may serve as landmarks for future 

 investigations. 



In the first beds of the Salins Limestone, immediately above the 

 Salins Marls, the Trigonia gibbosa, Sow., is quite abundant, and in a good 

 state of preservation. Fitton says, that the last bed in the Portland 

 quaiTies, called by the quarrymen " roach," contains a great quantity 

 of Trigonia gihbosa. So we may suppose that the " roach" of Portland 

 is equivalent to the first beds of the Salins Limestone, or a little older. 

 I have indicated both suppositions by dotted lines uniting the two 

 abstract sections. In both countries, the stratigraphical position of 

 the Hemicidaris Purbeclcensisy Forb., forbids the supposition that 

 the " roach " may be younger than the Salins " Trigonia gibbosa 

 beds." Until now, palaeontologists and geologists have regarded 

 the Echinodermata as more characteristic than the Acephala and 

 Gasteropoda, and of equal importance with the Cephalopoda and 

 Brachiopoda. As an example of their importance, it is sufficient to 

 say that Forbes replaced the Purbeck beds in the Jurassic rocks, because 

 he discovered a Hemicidaris in the ''Cinder-bed" near Swanage. 

 That Hemicidaris was new, and he called it Hemicidaris Purbeckensis. 



A few years later, the same species was signalized in France by 

 Cotteau, who had it from the Salins Limestone of Burgundy (see Etudes 

 sur les Echinides fossil es du departement de 1' Tonne, vol. i. p. 300). But 

 Cotteau says that his three specimens belong to a variety of the species 

 described by Forbes, who found only a single complete specimen ; and 



