6 
THE GEOLOGIST. 
I have given a rough sketch under the form of a tabular and 
proportional view (PI. I. Fig. 2) — Abstract Section of the series of 
strata comprised between the Portland stone and the Loiver Greensand, 
in the South-Eastern part of England — for the sake of comparison. 
The Bann6 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 Greensand, 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 Neocomian. 
A few marine fossils, or, at least, belonging to brackish- water 
animals, have lately been found common to the two series in the 
Jui'a 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 gibhosa, Sow., is quite abundant, and in a good 
state of preservation. Fitton says, that the last bed in the Portland 
quarries, 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 Uemicidaris . Purbeckensis, 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 Hemlcidaris 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 
8ur leg Echinides fossiles du departement de VYonne, 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 
