Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/398

Rh 376 MAMMALIA [MBSOZOIC MAMMALS. of the angular process is slightly inflected, and the mylo- hyoid groove persistent, as in some of the existing Mar supials and in Whalebone Whales. This groove, a remnant of that which originally lodges Meckel s cartilage, mistaken for a suture, was once considered evidence of the reptilian nature of these jaws. A second species is described as A. broderipii (Owen). B. Pkascolotherium, Owen, 1839. This is founded on a right ramus of the lower jaw, presenting the inner side to view. Its length is l - 4 inch. The number of teeth ale resemble those of existing insectivorous mammals, be ing i 3 or 4, c 1, p and m 7, but not clearly defined from each other. One spe cies, P. bucklandii (fig. 16). C. Stereognathus, Charles- worth, 1854. S. ooliticus _ // i i -i -, e FIG. 17. Stereognathus. Portion of Jaw, (fig. 1 /) IS founded On a frag- embedded in Oolitic matrix (nat. size). ment of a jaw of minute size From Oweu- with three molar teeth in situ. The grinding surface is of quadrate form, of very little height, and supports six sub- equal cusps. Its affinities are quite problematical. The freshwater bed previously alluded to, situated at Durdlestone Bay near Swanage, belongs to the Middle Purbeck series, intervening between the Middle Oolite and the Weald-en. The first discovery of mammalian bones was made in this spot by Mr W. R. Brodie in 1854, but the subsequent explorations of Mr S. H. Beckles have yielded a surprising number of species. They are associated with numerous saurians, insects, and freshwater shells, as Paludina, Planorbis, and Cyclas. No less than eleven genera from this locality alone are fully described in Professor Owen s memoir. These may be grouped as follows : A. With teeth arranged on the insectivorous type. Mandibular incisors more than two ; canines well de veloped ; premolars and molars cuspidate, seven or more : (a) molars and premolars more than eight, mostly twelve (Spa- size), Purbeck beds. From Owen. hicotherium, Amblotheriiim, Peralestes, Achyrodon, Pera- spalax, Peramus, Stylodon and Bolodon the last known only by the maxillary teeth); (b) molars and premolars seven or eight (Triconodon and Tria- canthodoii). As any synopsis of the characters of these genera would be scarcely intelligible Fin - w. Jaw of Triconodon .., . . * . . mordax (nat. size). From Owen. without minute descriptions and reference to figures, the reader who desires further infor mation is referred to the memoir cited above. B. With a single, strong, pointed, slightly curved incisor, placed close to the median line as in Rodents. No canine. Three or four compressed, trenchant, obliquely grooved premolars, increasing in size from first to last, and two small molars with low multituberculated crowns. Genus Plagiaulax, Falconer. This remarkable and highly specialized type has been the occasion of one of the most interesting discussions on the inferences which may be drawn as to the affinities and habits of an otherwise unknown animal from the structure of a small portion of its organization which occurs in the annals of natural history, a discussion carried on with great ability, ingenuity, and wealth of illustration on both sides. Dr Falconer maintained that it was more nearly allied to the Rat-Kangaroo (Hypsiprymnus) than to any other existing FIG. 18. Spalacotherium tricuspidens (twice nat. form, and that, as it is known that these animals feed upon grass and roots, &quot; it may be inferred of Playiaulax that the species were herbivorous or frugivorous. I can see nothing in the character of their teeth,&quot; he adds, &quot;to FIG. 21. Plagiaulax minor (four times nat. size). From Lycll. FIG. 20. Plagiaulax becclesii (twice nat. size). From Owen. indicate that they were either insectivorous or omnivorous.&quot; Professor Owen, on the other hand, from the same materials came to the conclusion that &quot; the physiological deductions from the above-described characteristics of the lower jaw and teeth of Plagiaulax are that it was a carnivorous Marsupial. It probably found its prey in the contemporary small insectivorous mam mals and Lizards, sup posing no herbivorous form like Stereognathus to have co-existed during the Upper Oolite period.&quot; It is impossible here to give at any length the arguments by which these opposing views are respectively supported, but it may be indicated that the first-mentioned is strongly countenanced by the consideration of the following facts : (1) all existing Marsupials may be divided, as far as their dentition is concerned, into two groups (a) those which have a pair of large more or less procumbent incisors close to the symphysis of the lower jaw, and rudimentary or no canines (diprotodont dentition ; families Phascolomyidse, Macropodidse, and Phalangistidse), and (b) those which have numerous small incisors, and large pointed canines (polyprotodont dentition; families Peramelidae, Dasynridx, and Didelphidse] ; (2) the vast majority of the former group are purely vegetable feeders, and almost all of the latter are carnivorous or insectivorous ; and (3) Plagiaulax, so far as its structure is known, belongs obviously to the former group, and, as we have no sure basis for inferences as to the habits of an unknown animal but the knowledge of the habits of such as are known, we have no grounds for supposing that its habits differed from those of its structural congeners. 1 That the two types of dentition still found among Marsupials should have existed side by side in so remote a period of time as that in which the Purbeck bone bed was deposited, and that one of these types should have already attained so singular a degree of specialization, is one of the most remarkable facts yet revealed by mammalian paleon tology. Whether the teeth of the upper jaw correspond also to the modern diprotodont type is a question of great interest, for the solution of which we must await future discoveries, of which we have more hope since the announce ment by Professor Marsh of the existence, in considerable numbers, of small mammals in the American Jurassic formations of the Rocky Mountains, which conform in all 1 The whole discussion is contained in the following memoirs : (1) H. Falconer, &quot; Description of Two Species of the Fossil Mammalian genus Plagiaulax, from Purbeck,&quot; Quart. Jour. Geol. Soc., August 1857 ; (2) R. Owen, art. &quot;Palaeontology,&quot; Encyclopaedia Britannica, 8th ed., 1859 ; (3) H. Falconer, &quot;On the Disputed Affinity of the Mammalian genus Plagiaulax,&quot; Quart. Jour. Geol. Soc., November 1 860 ; (4) R. Owen, Monograph of the Fossil Mammalia of the Mesozoic Formation,&quot; Palivontojraphical Society, 1871.