FOSSIL JAWS FROM STONESFIELD. 207 



Hum they progressively diminish, as in the Myrtnecobius. As the outer sides 

 of the grinders in the jaw of the Phascolotherium are imbedded in the ma- 

 trix, we cannot he sure that there is not a smaller cuspidated ridge sloping 

 down towards that side, as in the crowns of the teeth of the Myrtnecobius. 

 But, assuming that all the cusps of the teeth of the Phascolotherium are 

 exhibited in the fossil, still the crowns of these teeth resemble those of the 

 Thy lacy nus more than they do those of any placental Insectivora or Phoca, 

 if even the form of the jaw permitted a comparison of it with that of any of 

 the seal tribe. Connecting then the close resemblance which the molars of 

 the Phascolotherium bear to those of the Thylacynus with the similarities of 

 the ascending ramus of the jaw, Mr. Owen is of opinion that the Stonesfield 

 fossil was nearly allied to Thylacynus, and that its position in the marsupial 

 series is between Thylacynus and Didelphys. With respect to the supposed 

 compound structure of the jaw of the Phascolotherium, Mr. Owen is of 

 opinion that of the two linear impressions which have been mistaken for 

 harmonics or toothless sutures, one, a faint, shallow, linear impression, con- 

 tinued from between the antepenultimate and penultimate molars obliquely 

 downwards and backwards, to the foramen of the dental artery, is due to 

 the pressure of a small artery, and that the author possesses the jaw of a 

 Didelphys Virginiana, which exhibits a similar groove in the same place. — 

 Moreover, this groove in the Phascolotherium does not occupy the same re- 

 lative position as any of the contiguous margins of the opercular and den- 

 tary pieces of a reptile's jaw. The other impression in the jaw of the Phas- 

 colotherium is a deep groove, continued from the anterior extremity of the 

 fractured base of the inflected angle, obliquely downwards to the broken 

 surface of the anterior part of the jaw. Whether this line be due to a vas- 

 cular impression, or an accidental fracture, is doubtful; but as the lower 

 jaw of the wombat presents an impression in the precisely corresponding si- 

 tuation, and which is undoubtedly due to the presence of an artery, Mr. 

 Owen conceives that this impression is also natural in the Phascolotherium, 

 but equally unconnected with a compound structure of the jaw; for there 

 is not any suture in the compound jaw of a reptile which occupies a corre- 

 sponding situation. 



" The most numerous, the most characteristic, and the best marked su- 

 tures in the compound jaws of a reptile, are those which define the limits 

 of the coronoid, articular, angular, and surangular pieces, and which are 

 chiefly conspicuous on the inner side of the posterior part of the jaw. Now 

 the corresponding surface of the jaw of the Phascolotherium is entire, yet 

 the smallest traces of sutures, or of any indication that the coronoid or ar- 

 ticular processes were distinct pieces, cannot be detected ; these processes 

 are clearly and indisputably continuous, and confluent with the rest of the 

 ramus of the jaw. So that where sutures ought to be visible, if the jaw of 

 the Phascolotherium were composite, there are none ; and the hypothetical 

 sutures that are apparent, do not agree in position with any of the real su- 

 tures of an oviparous compound jaw. 



"Lastly, with reference to the philosophy of pronouncing judgment^ on 

 the saurian nature of the Stonesfield fossils, from the appearance of sutures, 

 Mr. Owen offered one remark, the justness of which, he said, would be obvi- 

 ous alike to those who were, and to those who were not, conversant with 

 comparative anatomy. The accumulative evidence of the true nature of 

 the Stonesfield fossils, afforded by the shape of the condyle, coronoid pro- 

 cess, angle of the jaw, different kinds of teeth, shape of their crowns, double 

 fangs, implantation in sockets, — the appearance, he repeated, presented by 

 these important particulars cannot be due to accident ; while those which 

 favour the evidence of the compound structure of the jaw, may arise from 

 accidental circumstances." 



