The _Muschelkalk_ consists of beds of compact limestone, often greyish, sometimes black, alternating with marl and clay, and commonly containing such numbers of shells that the name of shelly limestone (_Muschelkalk_) has been given to the formation by the Germans. The beds are sometimes magnesian, especially in the lower strata, which contain deposits of gypsum and rock-salt. The seas of this sub-period, which is named after the innumerable masses of shells inclosed in the rocks which it represents, included, besides great numbers of Mollusca, Saurian Reptiles of twelve different genera, some Turtles, and six new genera of Fishes clothed with a cuirass. Let us pause at the Mollusca which peopled the Triassic seas. [Illustration: Fig. 81.--Ceratites nodosus. (Muschelkalk.)] Among the shells characteristic of the Muschelkalk period, we mention _Natica Gaillardoti_, _Rostellaria antiqua_, _Lima striata_, _Avicula socialis_, _Terebratula vulgaris_, _Turbonilla dubia_, _Myophoria vulgaris_, _Nautilus hexagonalis_, and _Ceratites nodosus_. The _Ceratites_, of which a species is here represented (Fig. 81), form a genus closely allied to the _Ammonites_, which seem to have played such an important part in the ancient seas, but which have no existence in those of our era, either in species or even in genus. This Ceratite is found in the Muschelkalk of Germany, a formation which has no equivalent in England, but which is a compact greyish limestone underlying the saliferous rocks in Germany, and including beds of dolomite with gypsum and rock-salt. The _Mytilus_ or _Mussel_, which properly belonged to this age, are acephalous (or headless) Molluscs with elongated triangular shells, of which there are many species found in our existing seas. _Lima_, _Myophoria_, _Posidonia_, and _Avicula_, are acephalous Molluscs of the same period. The two genera _Natica_ and _Rostellaria_ belong to the Gasteropoda, and are abundant in the Muschelkalk in France, Germany, and Poland. Among the Echinoderms belonging to this period may be mentioned _Encrinus moniliformis_ and _E. liliiformis_, or _lily encrinite_ (Fig. 82), whose remains, constituting in some localities whole beds of rock, show the slow progress with which this zoophyte formed beds of limestone in the clear seas of the period. To these may be added, among the Mollusca, _Avicula subcostata_ and _Myophoria vulgaris_. In the Muschelkalk are found the skull and teeth of _Placodus gigas_, a reptile which was originally placed by Agassiz among the class of Fishes; but more perfect specimens have satisfied Professor Owen that it was a Saurian Reptile. It may be added, that the presence of a few genera, peculiar to the Primary epoch, which entirely disappeared during the sub-period, and the appearance for the first time of some other animals peculiar to the Jurassic period, give to the Muschelkalk fauna the appearance of being one of passage from one period to the other.