100 PKOF. OWEN ON A CARNIVOROUS REPTILE. 



dental tissue of each tooth, by retention in the dentine of sub- 

 parallel 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 con- 

 comitant 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 vegeta- 

 tive 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 struc- 

 ture 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," 

 11 molars," " canines," in the dental series, by characters of size, 

 shape, and relative position, with the same certainty, or on as satis- 

 factory grounds, as in the warm-blooded quadrupeds. With the 

 carnivorous type of incisors, canines, and molars we now have evi- 

 dence 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 Mar- 

 supialia and in the Ovo-viviparous Monotremes. 



In one of the species of Saurians with what is now the mam- 

 malian type of humerus and of dentition, evidence, for which I am 

 indebted to the Governor of the Cape of Good Hope, Sir IJenry 

 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 mam- 

 mals. Didelpliis, e. g., has i. ^jj as in Cynodraco ; Thylacinus 

 and Sarcojphilus have i. g^, as in Cynochampsa. In no placental 

 carnivore do the incisors exceed f 11 ? . 



o — o 



In the class of cold-blooded, air-breathing, naked Ovipara, as 

 known to zoology by existing species, the characters abo?e specified, 

 discussed, and compared are wanting. They would have been un- 



