Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/160

100 dental tissue of each tooth, by retention in the dentine of subparallel vascular canals, is associated a downward development of the zygomatic arch, for extending the origin and augmenting the size and force of the masseteric masticatory muscles.

This structure, unknown in any existing Reptiles, but exemplified in Iguanodon, Scelidosaurus, Pareiasaurus, reappears, so to speak, in certain Mammalia, and here in members of the class which by their low position according to cerebral characters, with genital and concomitant modifications, show a near approach to the cold-blooded Ovipara: the Sloth and Kangaroo are examples.

But cranial characters of lack, as of gain, are not wanting in this comparative glance. The lingering evidence, in Reptiles, of vegetative repetition, as manifested by multiplied centres of ossification in the facial blastema, has disappeared. The ectopterygoid ceases to exist in both Theri- and Dicynodonts; that bone never reappears in the mammalian series.

In no existing Reptilia is the principle of differentiation of structure adapting particular teeth for special functions exemplified as in the extinct Theriodontia. Not until the discovery of members of this order of Reptilia could the anatomist specify "incisors," "molars," "canines," in the dental series, by characters of size, shape, and relative position, with the same certainty, or on as satisfactory grounds, as in the warm-blooded quadrupeds. With the carnivorous type of incisors, canines, and molars we now have evidence of an associated humeral structure unknown in any lacertian or other existing reptilian fore limb, but a structure reappearing in certain mammals and notably in the implacental group of Marsupialia and in the Ovo-viviparous Monotremes.

In one of the species of Saurians with what is now the mammalian type of humerus and of dentition, evidence, for which I am indebted to the Governor of the Cape of Good Hope, Sir Henry Barkly, K.C.B., has reached me of the bony structure of the fore paw, which again shows an advance towards the mammalian type. The pollex has two phalanges; the four other digits have each three phalanges. The slight difference in length in these fingers is due, as in a dog's paw, to difference of length in such phalanges, not to difference of number of these, not to excess in the third and fourth digits beyond the number three, which number rules in the fingers of all terrestrial mammals.

Reverting to the chief character of the Theriodont reptiles, a fact of some significance may be noted, viz. that the incisive formula of some of the species is repeated in the low marsupial order of mammals. Didelphis, e.g., has i. $5—5⁄4—4$, as in Cynodraco; Thylacinus and Sarcophilus have i. $4—4⁄3—3$, as in Cynochampsa. In no placental carnivore do the incisors exceed $3—3⁄3—3$.

In the class of cold-blooded, air-breathing, naked Ovipara, as known to zoology by existing species, the characters above specified, discussed, and compared are wanting. They would have been un-