PLIOLOPHUS, Ow.—The most complete and instruc tive example of a Mammal from the next overlying division of the eocene tertiaries, viz., the " London clay," is that which the writer has described t under the name of Pliolophws vulpi ceps. It is a hoofed Herbivore, but presents a dentition not exhibited by any later or existing species of Mammal.
The length of the skull (fig. 94) is 4 inches, its extreme Skull of Pliolophus vuipiceps (half nat. size), London clay.
breadth 2 inches 2 lines, the height of the cranium opposite the first premolar tooth 9 lines. Its shape and characteristics determine the hoofed nature of this species, and its affinities • to the Perissodactyla, or the order of Ungulata with toes in odd number. The extent and well-defined boundary of the tem poral fosse; by the occipital ( 3 ), parietal ( 7 ), and post-frontal ridges, and their free communication with the orbits, give almost a carnivorous character to this part of the cranium of Pliolophus; but as in the hog, Hyrax, and Palteothere, the greatest cerebral expansion is at the middle and toward the fore part of the fossae, with a contraction toward the occiput ; the brain-case not continuing to enlarge backward to beyond the origin of the zygomata, as in the fox. The zygomatic arches have a less outward span than in the Carnivore. In this part of the cranial structure Pliolophus resembles Palizo therium more than it does any existing Mammal ; but the post-frontal processes are longer and more inclined backward. The incompleteness of the orbit occurs in both Anoplotheriuna and Palceotherium, as in Rhinoceros, Tapirus, and the hog tribe ; but in the extent of the deficient rim, Pliolophus is inter mediate between Palteotherium and Tapirus. The orbit is not so low placed as in Padeeotherium, Tapirus, and .Rhinoceros, nor so high as in Hyrax or Sus. The straight upper contour of the skull (7 to is) is like that in the horse tribe and Hyrax, and differs from the convex contour of the same part in the Anoplothere and PaIxothere. The size of the antorbital fora men (a) indicates no unusual development of the muzzle or upper lip. In the conformation of the nasal aperture by four bones (two nasals, is, and two premaxillaries, 22), Pliolophus resembles the horse, Hyrax, hog tribe, and Anoplothere, and differs from the rhinoceros, tapir, and Palteothere, which have the maxillaries, as well as the nasals and premaxillaries, enter ing into the formation of the external bony nostril.
The ungulate and herbivorous character of Pliolophus is most distinctly marked by the modifications of the lower jaw, especially by the relative dimensions of the parts of the ascend ing ramus which give the extent of attachment of the biting (temporal) and grinding (masseteric and pterygoid) muscles respectively. In the shape of the mandible Pliolophus most resembles Tapirus among existing Mammals, and the Palm theriwra among the extinct ones in which that shape is known. As in almost every species of eocene quadruped yet discovered, the Pliolophus presents the type-dentition of the placental diphyodont series, viz.— The incisors are preserved in the lower jaw with marks of attrition on their crowns demonstrating corresponding teeth of the same number (six), and of similar size, in the upper jaw, from which the alveolar part of the premaxillaries had been broken away.
The canines are small in both jaws : they are separated by a vacant space from the outer incisors, and by a longer inter val from the first premolars. These form a continuous series with the remaining teeth in the upper jaw, but are separated by a space of about half their breadth from the second pre molar in the lower jaw. The succeeding teeth (p z, 3, 4) increase in size to the penultimate molar in the upper, and to the last molar in the lower jaw ( p 4, in figs. 95 and 96), which tooth has a third lobe.
In the last premolar upper jaw (fig. 95,p 4) the cingulum is uninterrupted along the outer side from its anterior well developed talon (c) to the back part. The two outer cones resemble those of the true molars; but there is only one inner cone, and the crown of p 4 differs accordingly from that of m 1, in being triangular rather than square. A ridge is con tinued from the interspace between the anterior talon (c), and the outer anterior lobe obliquely inward and backward to the inner lobe, swelling into a small tubercle at the middle of its course.