PLAGIAULAX, * Fr.—The most remarkable of Mr. Beccles' discoveries in the above formation are the mammalian jaws indicative of the genus above named, of which two species have been determined by Dr. Falconer.
Sp. Plagiaulax Becclesii, Fr.—Two specimens exemplified the shape and pro portions of the entire jaw of this species (fig. 92). The fore most tooth (i) is a very large one, shaped like a canine, but implanted by a thick root in the fore part of the jaw, like the large lower incisor of a shrew or wombat. The three anterior teeth in place have compressed trenchant crowns, and rapidly augment in size from the first ( 2.) to the third (4). They are followed by sockets of two much smaller teeth, shown in other specimens to have sub tuberculate crowns resembling those of Microlestes. The large front tooth of Plagiaulax is formed to pierce, retain, and kill ; the succeeding teeth, like the carnassials of Carnivora, are, like the blades of shears, adapted to cut and divide soft substances, such as flesh. As in Carnivora, also, these sectorial teeth are succeeded by a few small tubercular ones. The jaw conforms to this character of the dentition. It is short in proportion to its depth, and consequently robust., sending up a broad and high coronoid process (b), for the adequate grasp of a large temporal muscle ; and the condyle (c) is placed below the level of the grinding teeth—a character unknown in any herbivorous or mixed-feeding Mammal ; whilst the lever of the coronoid process is made the stronger by the condyle being carried farther back from it than in any known carnivorous or herbivorous animal. The angle of the jaw makes no projec tion below the condyle, but is slightly bent inward, according to the marsupial type.
Sp. Plagiaulax minor, Fr.—In this species the first premolar (fig. 93, p, 1) is preserved ; the rest (p, 2, 3, and 4) show nearly the same shape and propor tions as in P. Beedesii. The first molar (m, 1) has a broad depression on the grinding surface, surrounded by tuber cles, of which three are on the outer border ; the marginal tubercles of the second smaller tooth are smaller and more numerous.
In the general shape and proportions of the large premolar (p, 4) and succeeding molars, Plagiaulax most resembles Thylaeoleo (fig. 141, p, rn,, z and 2),—a much larger extinct predaceous Marsupial from tertiary beds in Australia. But
the sectorial teeth in Plagiaulax are more deeply grooved ; whence its name. The single compressed premolar of the kangaroo-rat is also grooved ; but it is differently shaped, and is succeeded by four square-crowned double-ridged grinders adapted for vegetable food ; and the position of the condyle, the slenderness of the coronoid, and other characters of the lower jaw, are in conformity to that regimen. In Thylacoleo the lower canine or canine-shaped incisor projected from the fore part of the jaw close to the symphysis, and the correspond ing tooth in Plagiaulax more closely resembles it in shape and direction than it does the procumbent incisor of Hypsiprymnus. From this genus Plagiaulax differs by the obliquity of the grooves on its premolars ; by having only two true molars in each ramus of the jaw, instead of four ; by the salient angle which the surfaces of the molar and premolar teeth form, instead of presenting a uniform level line ; by the broader, higher, and more vertical coronoid ; and by the very low position of the articular condyle.
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 Mammals and Lizards, supposing no herbivorous form, like Stereognathus, to have co-existed during the upper oolitic period.
In the Woodwardian Museum at Cambridge is a specimen of anchylosed cervical vertebrae of a cetaceous animal as large as a grampus, but presenting specific distinctions from all known recent and fossil species. It is stated to have been found in the brown clay or "till" near Ely ; but in its petri fied condition, colour, and specific gravity, it is so different from the true bones of the "till," and so closely like the fossils of the Kimmeridge clay, as to make it extremely probable that it has been washed out of that formation.
No evidence of the mammalian class has yet been met with in the chalk beds.
The examples of the Mammalia first met with in tertiary strata are the Coryphodon and Paiceocyon, respectively repre senting the ungulate (herbivorous) and unguiculate (carnivor ous) modifications of the class ; their remains have been found in the plastic clay and equivalent lignites in England and France.