Mr. Owen on the Phascolotherium. 61 



mistake is kept in view, the anatomist, anxious after truth, would pause before he 

 accepted the reputed structure of the molars of the Basilosaurus as valid proof of 

 so anomalous an exception to the condition of the teeth characteristic of the class 

 Reptilia, if even the Basilosaur should prove to be a true Saurian. In the teeth 

 of the Megatherium the appearance of the double fang was produced by an ac- 

 cidental fracture in the axis of the tooth, displaying the two converging sides 

 of the simple conical cavity, which, as in the teeth of the larger Saurians, occupies 

 the base of the really simple and fangless tooth of the Megathere. 



Until, therefore, the teeth of the Basilosaurus shall have been examined with a 

 view to the possibility of this mistake, and their size, form and relations to the 

 jaws have been described, the simple statement that such an extinct animal had 

 teeth with double fangs, can have little force against the argument for the mara- 

 miferous nature of the Thylacothere, founded on the structure of its teeth*. 



The objection to the mammiferous character of the Thylacothere founded on 

 the colour of the fossil, as indicative of a proportion of animal matter originally 

 therein present, and presumed to be such as is found only in the cold-blood Verte- 

 brata, will have but little weight with the geologist practically acquainted with 

 fossil remains. The dentine of the Mastodon's grinders commonly exhibits a 

 colour much more nearly approaching to jet black than is presented by any tooth 

 of the Thylacothere. On the other hand, there are numerous fossils of both reptiles 

 and fishes, which, from the accident of their matrix, offer evidence of a much less 

 original proportion of the animal matter than is conjecturally assigned to those 

 of the Thylacothere. 



There remain then for discussion the arguments for the saurian nature of 

 the Stonesfield fossils, founded on the appearance of a compound structure ob- 

 servable in them. 



As these appearances are more numerous and imposing in the jaw of the Phas- 

 colotherium about to be described, I have reserved their consideration to the 

 present opportunity, repeating only, with respect to the Thylacothere, that the 

 only trace of this structure in the jaw of that fossil is due to a mere vascular 

 groove running along its lower margin ; and that a similar structure is present 

 in the lower jaw of some species of the Opossum, in the Wombat, Myrmecobius, 

 and in that of the Sorex Indicus and of other Mammalia. 



Description of the Half-jaw of Phascolotherium Bucklandi. PI. VI. fig. 2. 

 The fossil on which this genus is founded is the right ramus of the lower jaw, having its external 



* Since this paper was read, I have had the unexpected gratification of being able to determine the 

 mammiferous, and probably cetaceous nature of the so-called Basilosaurus. See posted, p. 69 of the 

 present volume. 



