if the bipedal mode of progression was more than an occasional thing amongst the Deinosaurs, and the greater number of the many known tracks exhibit no impressions of fore-feet. Upon the whole, therefore, we may, with much probability, conclude that the great class of Birds (_Aves_) was in existence in the Triassic period. If this be so, not only must there have been quite a number of different forms, but some of them must have been of very large size. Thus the largest footprints hitherto discovered in the Connecticut sandstones are 22 inches long and 12 inches wide, with a proportionate length of stride. These measurements indicate a foot four times as large as that of the African Ostrich; and the animal which produced them--whether a Bird or a Deinosaur--must have been of colossal dimensions. [Illustration: Fig. 156.--Lower jaw of _Dromatherium sylvestre_. Trias, North Carolina. (After Emmons.)] [Illustration: Fig. 157.--a, Molar tooth of _Micro estes antiquus_, magnified; b, Crown of the same, magnified still further. Trias, Germany.] [Illustration: Fig. 158.--The Banded Ant-eater (_Myrmecobius fasciatus_) of Australia.] Finally, the Trias completes the tale of the great classes of the Vertebrate sub-kingdom by presenting us with remains of the first known of the true Quadrupeds or _Mammalia_. These are at present only known by their teeth, or, in one instance, by one of the halves of the lower jaw; and these indicate minute Quadrupeds, which present greater affinities with the little Banded Anteater (_Myrmecobius fasciatus_, fig. 158) of Australia than with any other living form. If this conjecture be correct, these ancient Mammals belonged to the order of the Marsupials or Pouched Quadrupeds (_Marsupialia_), which are now exclusively confined to the Australian province, South America, and the southern portion of North America. In the Old World, the only known Triassic Mammals belong to the genus _Microlestes_, and to the probably identical _Hypsiprymnopsis_ of Professor Boyd Dawkins. The teeth of _Microlestes_ (fig. 157) were originally discovered by Plieninger in 1847 in the "bone-bed" which is characteristic of the summit of the Rhætic series both in Britain and on the continent of Europe; and the known remains indicate two species. In Britain, teeth of _Microlestes_ have been discovered by Mr Charles Moore in deposits of Upper Triassic age, filling a fissure in the Carboniferous limestone near Frome, in Somersetshire; and a molar tooth of _Hypsiprymnopsis_ was found by Professor Boyd Dawkins in Rhætic marls below the "bone-bed" at Watchet, also in Somersetshire. In North America, lastly, there has been found in strata of Triassic age one of the branches of the lower jaw of a small Mammal, which has been described under the name of _Dromatherium sylvestre_ (fig. 156). The fossil exhibits ten small molars placed side by side, one canine, and three incisors, separated by small intervals, and it indicates