Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/397

Rh MESOZOIC MAMMALS.] MAMMALIA 375 MAMMALS OF THE MESOZOIC PERIOD. l The hitherto discovered remains of mammals which ex isted anterior to the Tertiary period all belong to creatures of very small size, the largest scarcely exceeding the common Polecat or Squirrel. Some are known only by a few isolated teeth, others by nearly complete sets of these organs, and the majority by more or less perfect specimens of the rami of the lower jaw. It is a very curious circumstance that this part of the skeleton alone has been preserved in such a large number of instances. No complete cranium has ever been found, nor is there satis factory evidence of the structure of the vertebral column or of the limbs of any single individu.il. The species already described from European strata amount to nearly thirty, which have been arranged in fifteen genera. Of these by far the greater number have been found at a single spot near Swan age in Dorsetshire, in a bed of calcareous mud only 40 feet long, 10 feet wide, and averaging 5 inches in depth, The marvellous results obtained by the exploration by Mr S, H. Beckles of this small fragment of the earth s surface show by what accidents, as it were, our knowledge of the past history of life has been gained, and what may still remain in store where little thought of at present. A bed, apparently equally rich, has recently been discovered in the Territory of Wyoming, North America, the contents of which are being made known by Professor Marsh. 1. Mammals of the Triassic Period. The Rhajtic formations, so named from the Rhaetian Alps of Bavaria, are the highest beds of the Trias, and are situated above the New Red Sandstone, and just below the Lia^. In 1847 Professor Pleininger of Stuttgart, while assiduously sifting some sand from this formation, belonging to the Keuper of Diegerloch and Steinenbronn, discovered, among an immense mass of teeth, scales, and unrecognizable fragments of skeletons of fish and saurians, two minute teeth, each with well-defined, enamelled, tuberculated crowns and distinct roots, plainly showing their mammalian character. These, the oldest known evidence of the class, were considered by their discoverer to indicate a predaceous and carnivorous animal of very small size, to which he gave the name of Micrulestes antiquus. Subsequently Mr C. Moore discovered in a bone bed of Rhsetic age filling a fissure in the mountain limestone at Holwell, near Frome in Somersetshire, various isolated teeth with their crowns much worn, but apparently including both upper and lower molars and a canine, which are assigned by Professor Owen to Pleininger s genus Microlestes, and described specifically as M. moorei. Under the name of Hypsi- prymnopsis rhxticus Prof. Boyd Dawkins 2 has described a single tooth with two roots which he discovered in a RhcBtic marlstone at Watchet in Somersetshire, and which may be even somewhat older than the last. Professor Dawkins finds tha nearest analogue of this tooth among recent mammals in the large trenchant premolar of the Rat-Kangaroo or Hypsiprymnus (see vol. xiii. p. 840, fig. 4), a resemblance not concurred in by Professor Owen, who refers it to the genus JMicrolestex. The minute size and worn condition of the tooth render it extremely difficult to form a decided opinion upon its characters, and therefore upon the affinities of the animal to which it belonged. Still more satisfactory evidence of the presence of mammals at a period at least as ancient as the European Trias is afforded by the discovery of three nearly perfect 1 The subjects referred to under this heading are mostly described and figured in detail in Owen s &quot; Monograph of the Fossil Mammalia of the Mesozoic Formations, &quot; Palfeontoyraphical Society s Publications, 1871; and in various papers by Marsh, in the American Journal of Science and Arts, 1878-80. 2 Quart. Jour. Geol. Soc., vol. xx. p. 411, 1864. mandibular rami in the Chatham coal-field of North Carolina by Dr Emmons, who, however, placed them as far back in age as the Permian, or altogether beyond the Mesozoic stage, a conclusion not now received. Of this animal, called Dromatheriam sylvestre, the complete dentition of the lower jaw is known, and consists of three pointed incisors, separated by intervals, one canine, and ten molars, of which the first three have simple sub- compressed crowns, and the remainder are multicuspid. The jaw figured by Dr Emmons 3 is -^ of an inch in length. He considered it to belong to a placental Insectivore, but the number of molar teeth exceeds that of any existing member of that order, and is only found in some Marsupials. It was associated in the same bed with thecodont reptiles. 2. Mammals of the Jurassic Period.- In the ascending order of geological age the next remains of mammals have been met with in the Lower Oolite at Stonesfield in Oxfordshire, where they are associated with wing cases of insects, Plesiosaurs, Crocodiles, and Pterodactyles. From this bed several specimens have been met with at various times, which have been placed in three genera. A. Amphitherium, Blainville, 1838. The specimen (A. prevostii, see fig. 15) upon which this genus was founded, was discovered in 1812, and examined in 1818 at Oxford by Cuvier, who pronounced it to be mammalian, and to resemble the jaw of an Opossum. This conclusion was afterwards disputed by De Blainville and others, who FIG. 15. Lower Jaw and Teeth of Amphitherium prevoftii (twice nat. tizi). From Owen. believed it to be reptilian, but the original determination is now generally accepted. 4 Thiee rami of mandibles, all more or less perfect, are now known. The length of the jaw is rather less than an inch. It contains sixteen teeth, which, as defined by shape only, are i 3, c 1, p 6, m 6, so that if the upper jaw had a corresponding number FIG. 1C. Lower Jaw and Tie h of Phascolotherinm bucktandii (nat. size in outline). From Owen. there would be sixty-four teeth in all, a greater number than in any existing heterodont mammal, though equalled by some of the species from the Purbeck. Ihe nearest approach to this number is in Myrmecobius among recent Marsupials. The incisors are rather long and slender, the canines apparently not much larger than the incisors, all the premolars and molars two-rooted the former with a single large pointed cusp and small basal cusp on one or both sides, the latter quinquecuspidate. The lower margin 3 American Geology, part vi. , p. 93, 1857. 4 A lull description of this interesting fossil, with a history of the discussion regarding its nature, is giveii in Owen s British Fossil Mammals and Birds.