FOSSIL JAWS FROM STONESFIELD. 205 



that lie had sought in the next place for secondary characters which might 

 reveal the group of Mammalia to which the remains could be assigned, and he 

 had found in the modification of the angle of the jaw, combined with the form, 

 structure, and proportions of the teeth, sufficient evidence to induce him to 

 believe that the Thylacotherium was a marsupial quadruped. 



"Mr. Owen then recapitulated the objections against the mammiferous 

 nature of the thylacotherian jaws, from their supposed imperfect state, and 

 repeated his former assertion, that they are in a condition to allow of these 

 characters being fully ascertained : he next reviewed, first, the differences 

 of opinion with respect to the actual structure of the jaw ; and secondly, to 

 the interpretation of admitted appearances. 



"1. As respects the structure. — It has been asserted that the jaws must 

 belong to cold-blooded Vertebrata, because the articular surface is in the 

 form of an entering angle ; to which Mr. Owen replies that the articular 

 surface is supported on a convex condyle, which is met with in no other 

 class of Vertebrata except in the Mammalia. Again, it is asserted that the 

 teeth are all of an uniform structure, as in certain reptiles ; but, on refe- 

 rence to the fossils, Mr. Owen states it will be found that such is not the 

 case, and that the actual difference in the structure of the teeth strongly 

 supports the mammiferous theory of the fossils. 



"2. With respect to the argument founded on an interpretation of struc- 

 ture which really exists, the author showed that the Thylacotherium having 

 eleven molars on each side of the lower jaw, is no objection to its mammi- 

 ferous nature, because among the placental Carnivora, the Canis Megalotis 

 has constantly one more grinder on each side of the lower jaw than the usual 

 number ; because the Chrysochlore among the Insectivora has also eight in- 

 stead of seven molars in each ramus of the lower jaw; and the Myrmeco- 

 bius, among the Marsupialia, has nine molars on each side of the lower jaw ; 

 and because some of the insectivorous armadillos and zoophagous Cetacea 

 offer still more numerous and reptile-like teeth, with all the true and essen- 

 tial characters of the mammiferous class. The objection to the false mo- 

 lars having two fangs Mr. Owen showed was futile, as the greater number 

 of the spurious molars in every genus of the placental Ferce have two fangs, 

 and the whole of them in the Marsupialia. If the ascending ramus in the 

 Stonesfield jaws had been absent, and with it the evidence of their mam- 

 miferous nature afforded by the condyloid, coronoid and angular processes, 

 Mr. Owen stated that he conceived the teeth alone would have given suffi- 

 cient proof, especially in their double fangs, that the fossils do belong to 

 the highest class of animals. 



" In reply to the objections founded on the double fangs of the Basilo- 

 saurusy Mr. Owen said that the characters of that fossil not having been 

 fully given, it is doubtful to what class the animal belonged ; and, in an- 

 swer to the opinion that certain sharks have double fangs, he explained that 

 the widely bifurcate basis supporting the tooth of the shark, is no part of 

 the actual tooth, but true bone and ossified parts of the jaw itself, to which 

 the tooth is anchylosed at one part, and the ligaments of connexion attach- 

 ed at the other. The form, depth and position of the sockets of the teeth 

 in the Thylacotherium are precisely similar to those in the small opossums. 

 The colour of the fossils, Mr. Owen said, could be no objection to those 

 acquainted with the diversity in this respect, which obtains in the fossil re- 

 mains of Mammalia. Lastly, with respect to the Thylacotherium, the au- 

 thor stated that the only trace of compound structure is a mere vascular 

 groove running along its lower margin, and that a similar structure is pre- 

 sent in the corresponding part of the lower jaw of some species of opossum, 

 of the wombat, of the Balcena antarctica, and of the Myrmecobius, though 

 the groove does not reach so far forward in this animal ; and that a similar 



