Beds of Passage between the Lias and Trias—Rhætic Beds.—We have mentioned in the last chapter (p. 356) that the base of the Lower Lias is characterised, both in England and Germany, by beds containing distinct species of Ammonites, the lowest subdivision having been called the zone of _Ammonites planorbis._ Below this zone, on the boundary line between the Lias and the strata of which we are about to treat, called “Trias,” certain cream-coloured limestones devoid of fossils are usually found. These white beds were called by William Smith the White Lias, and they have been shown by Mr. Charles Moore to belong to a formation similar to one in the Rhætian Alps of Bavaria, to which Mr. Gumbel has applied the name of Rhætic. They have also long been known as the Koessen beds in Germany, and may be regarded as beds of passage between the Lias and Trias. They are named the Penarth beds by the Government surveyors of Great Britain, from Penarth, near Cardiff, in Glamorganshire, where they sometimes attain a thickness of fifty feet. The principal member of this group has been called by Dr. Wright the _Avicula contorta_ bed,[1] as this shell is very abundant, and has a wide range in Europe. General Portlock first described the formation as it occurs at Portrush, in Antrim, where the _ Avicula contorta_ is accompanied by _Pecten Valoniensis,_ as in Germany. The best known member of the group, a thin band or bone-breccia, is conspicuous among the black shales in the neighbourhood of Axmouth in Devonshire, and in the cliffs of Westbury-on-Severn, as well as at Aust and other places on the borders of the Bristol Channel. It abounds in the remains of saurians and fish, and was formerly classed as the lowest bed of the Lias; but Sir P. Egerton first pointed out, in 1841, that it should be referred to the Upper New Red Sandstone, because it contained an assemblage of fossil fish which are either peculiar to this stratum, or belong to species well-known in the Muschelkalk of Germany. These fish belong to the genera _Acrodus, Hybodus, Gyrolepis,_ and _Saurichthys._ Fig. 383: Cardium rhæticum. Fig. 384: Pecten Valoniensis. Fig. 385: Avicula contorta. Fig. 386: Hybodus plica ilis. Fig. 387: Saurichthys apicalis. Fig. 388: Gyrolepsis tenuistriatus. Among those common to the English bone-bed and the Muschelkalk of Germany are _Hybodus plicatilis_ (Fig. 386), _Saurychthys apicalis_ (Fig. 387), _Gyrolepis tenuistriatus_ (Fig. 388), and _G. Albertii._ Remains of saurians, _Plesiosaurus_ among others, have also been found in the bone-bed, and plates of an _Encrinus._ It may be questioned whether some of those fossils which have the most Triassic character may not have been derived from the destruction of older strata, since in bone-beds, in general, many of the organic remains are undoubtedly derivative.