long upper tusks in exemplifying Mammalian characters, not Lacertian ones. The same concurrence, not only with well-defined canines, but likewise with equally well-defined incisors and molars, has been observed by me in several species of extinct Reptilia from the Karoo deposits of South Africa.
The earliest examples (Galesaurus, Cynochampsa) of such Reptilia were made known through this Society by its 'Quarterly Journal,' vol. xvi. (for the year 1859–60). Since that date I have had evidences of the genera Lycosaurus, Tigrisuchus, Cynosuchus, Nythosaurus, Scaloposaurus, Procolophon, Gorgonops, as well as of the genus Cynodraco, of one species of which the dental and humeral characters have supplied the chief subject of the present communication.
Most of the genera above mentioned are represented by more than one species[1]: and both genera and species are classifiable in groups characterized by modifications of the structure of the external bony nostrils—as, for example, into the "tectinarial," "binarial," and "mononarial" families, the skull in the latter presenting, in the aspect and position of the single terminal nostril, a strikingly mammalian facies. Por the name of these extinct carnivorous Saurians I find it convenient, and believe it will be generally acceptable, to form a distinct order of Reptilia under the denomination of Theriodontia, with the following characters:—"Dentition of the carnivorous type; incisors defined by position and divided from the molars by a large laniariform canine on each side of both upper and lower jaws, the lower canine crossing in front of the upper; no ectopterygoids; humerus with an entepicondylar foramen; digital formula of fore foot 2, 3, 3, 3, 3 phalanges."
When we contrast the grand development and notable modifications of the extinct species of Reptilia with the poor scanty evidences of the class still lingering on in life, we seem to be witnessing a course of degeneration, of retrogradation, rather than of organic progression in the course of time.
Although there be none now, there have been Saurians in which the articular surfaces of certain vertebræ in the same column were modified—as, e.g., of the anterior ones in great herbivorous Dinosaurs, for freer movements of the head, by means of an anterior ball playing in a posterior cup, such as we now find in the Rhinoceros and some other large mammalian herbivores. In another part of the same vertebral column of these extinct reptiles we find a sacrum not limited to two, but composed of five or six ankylosed vertebræ, also as in these and many other mammals.
Again, some of these vegetarian Dinosaurs had masticatory teeth of a complex structure[2] unknown in existing phytophagous Sauria, but resembling those teeth of certain phytophagous mammals, e.g. Megatherium[3], Mylodon[4]. With the modification of a portion of the
