According to lists prepared by Mr. Etheridge for publication in a forthcoming work, the British species in undoubted Rhaetic beds may be summarized as follows, omitting the Sutton species. For the substance of the remarks regarding affinities and distribution of the species named I am also indebted to Mr. Etheridge.
Foraminifera, thirteen genera and twenty-seven species ; Crustacea 2, viz. Tropifer loevis from one of the bone-beds, and Estheria minuta, first known in the Keuper Sandstones ; one Brachiopod, Discina Townshendi, the only one known in our Rhaetic strata. Of the forms named in this list, Lima proecursor somewhat resembles L. punctata of the Lias ; Monotis decussata occurs at the very top of the Rhaetic beds in thin limestone bands, which some writers consider to form the bottom of the Lower Lias. Ostrea fimbriata, may possibly be O. irregularis of the Lias ; but oysters are of little value in such an inquiry. Pecten valoniensis, also a true Rhaetic shell, is a very variable form. Plicatula interstriata probably passes into the Lower Lias. Anoplophora musculoides, another true Rhaetic shell, also occurs with Monotis decussata in the thin limestone bands at the top, which some call Lias. Mytilus minutus occurs both in the Rhaetic and Lower Lias strata. All the Gasteropoda of the Rhaetic beds (not including the Sutton species) are peculiar to that formation ; and the same is the case with the fish. Of the Reptilia, Plesiosaurus costatus, P. Hawkinsii, and P. trigonus are common to the Rhaetic beds and the base of the Lower Lias*. The occurrence of Microlestes antiquus in the Keuper Marls is an accident, its remains having been washed into the strata from the neighbouring land†. Of the whole, not more than four species of shells at the most pass into the Lias ; and probably this may even be restricted to two. The Saurians have a longer range ; and this is very significant.
In this list I do not include the Sutton forms given in Mr. Etheridge's list, fourteen in all, one of which (Ostrea multicostata) is also found in the Muschelkalk, and three of which (Pecten Etheridgi, P. suttonensis, and Mytilus minutus) are also Lias species. Few or none of the remaining forms occur associated with the shells of the ordinary Rhaetic areas. The Sutton beds he unconformably on the Carboniferous Limestone, and stratigraphically and lithologically are inseparable from the ordinary Lias limestone. The corals, which Dr. Duncan has examined, occur in small, irregular, broken layers, or rather in occasional white tufaceous limestone patches, at the very
- Since this paper was written, I have been informed by Mr. William Sanders,
of Bristol, that he has obtained the centrum of an Ichthyosaurus of vast size "in close contact with the thick bone-bed at Aust Passage," apparently identical with I. platyodon of the Lower Lias.
† The Mendip Hills and the highlands of South Wales have been recognized as land while the Trias was being deposited, ever since 1846, if not earlier. Sir H. De la Beche (" On the formation of the Rocks of South Wales and Southwest of England," vol. i. p. 239 et seq., Mem. Geol. Surv. of Great Britain) considers the dolomitic conglomerates "on the Mendip Hills, for instance," "to have been beaches among islands." See also Ramsay, ' Denudation of S. Wales,' &c. p. 318. Mr. Moore also adopts the island theory, " Abnormal Secondary Deposits," Quart. Journ. Geol. Soc. 1867, vol. xxiii. p. 454.
