Dentalium 1, Natica 2, Pleurotomaria 3, Rissoa 1, Straparolus 1, Turbo 5, Turbonilla 4 : in all, 11 genera and 26 species.
Pteropoda. Theca 1.
Cephalopoda. Nautilus 1.
The whole comprises only 38 genera and 80 species.
All of these are small and dwarfed in aspect, when compared with their Carboniferous congeners, when such there are.
In this poverty and dwarfing of the forms, these Magnesian-Limestone fossils may also be compared with the still less numerous fauna of the Caspian ; and though I am not aware that that inland sea contains any Corals or Polyzoa, yet I doubt if the presence of two or three species of Chotetes (3) and Polycoelia (2), together with half a dozen Polyzoa and a very small Cyathocrinus, would entitle us to assume that it is impossible that they might have lived in an inland salt lake, which, like the Caspian, had previously been connected with the open ocean.
The Magnesian-Limestone series of the east of England may possibly, however, have been connected directly with an open sea at the commencement of the deposition of these strata, whatever its subsequent history may have been ; for the fish of the Marl-slate have generically strong affinities with those of Carboniferous age, some of which were undoubtedly truly marine, while others certainly penetrated shallow lagoons bordered by peaty flats. But the Marl-slate fish afford no certain clue to the solution of the problem as to whether in our area and in other parts of Europe they inhabited open sea or isolated inland salt waters. Indeed there is much to be said on the other side of the question from the reptiles found by Messrs. Howse and Hancock* ; for the Lepidotosaurus Duffii, which was found very near the base of the limestone in marly limestone passing down into Marl-slate, was a Labyrinthodont Amphibian, and Proterosaurus Speneri and P. Huxleyi, from the Marl-slate, were Lacertilian Reptiles.
Besides the poverty and small size of the Mollusca, the later strata of the true Magnesian Limestone seem to me to afford strong hints that they may have been deposited in a great inland salt lake subject to evaporation. Mr. Sorby, in a paper read before the British Association in 1856, considers that the Permian dolomite was chiefly " derived from comminuted and decayed calcareous organisms, and subsequently altered into dolomite," and " that probably this alteration was effected by the infiltration of the soluble magnesian salts of the sea- water, under conditions not yet clearly explained, during the period when it became so far concentrated that rock-salt was frequently deposited, and that the calcareous salt removed during the change had, by decomposition with the sulphates of the sea-water given rise to the accumulations of gypsum." Gypsum is common in the red marls of the Permian strata and in
- " On a new Labyrinthodont Amphibian from the Magnesian Limestone of
Midderidge, Durham," and " On Proterosaurus Speneri and P. Huxleyi from the Marl-slate of Midderidge, Durham," by Albany Hancock and Richard Howse (Quart. Journ. Geol. Soc. 1870, vol. xxvi. pp. 556 & 565).
