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FOSSIL MAMMALIA. 



17 



scale, proposed by Daubenton to determine the diversities of the form of the 

 cranium, as a gage of the intelligence of different animals* ; and the indication 

 of the limited capacity of the Toxodon, thus afforded, is strengthened by the 

 very small proportion, which the bony walls of the cerebral cavity bear to the zygo- 

 matic and maxillary parts of the skull, and to the size of the vertebral column, 

 as indicated by the condyloid processes, and foramen magnum. 



The zygomatic arches are of remarkable size and strength ; they commence 

 immediately anterior to the sides of the occipital plane, increase in vertical extent 

 as they pass outwards, forwards and downwards, and are suddenly contracted as 

 they bend inwards to abut against the sides of the sockets of the two posterior 

 molar teeth. 



The cranial cavity is remarkably narrow at the space included by the zygo- 

 matic arches ; being, as it were, excavated on each side to augment the space for 

 the lodgment of the temporal muscles, so that its diameter at this part is less than 

 that of the anterior extremity of the upper jaw. The upper surface of the cranium 

 expands to form the post-orbital processes, and again contracts anterior to these. 



The muscular ridges, or other characters, at the top of the skull, cannot be 

 precisely determined, as a great proportion of the outer table of the bone is broken 

 away, exposing a coarse and thick diploe. There seems, however, to have been 

 a strong ridge separating the occipital from the coronal or upper surface of the 

 cranium. The form of the remaining parts, which are modified in relation to 

 the attachment of the muscles of the jaws, indicates that these were powerfully 

 developed both for the offices of mastication and prehension. The general form 

 of the skull, while it presents certain points of resemblance to that of the aquatic 

 Pachydermata, and even of the Carnivora, has much that is peculiar to itself; but, 

 in the facial part, it approaches the nearest to that of the Rodentia ; and the den- 

 tition of the Toxodon, as exhibited in the upper jaw, corresponds with that which 

 characterizes the Rodent Order. 



The teeth of the Toxodon consist of molars and incisors, separated by a long 

 diastema, or toothless space. In the upper jaw the molars are fourteen in number, 

 there being seven on each side ; the incisors four, one very large, and one small, 

 in each intermaxillary bone. 



The general form and nature of the teeth are indicated by the sockets; and the 

 structure of the grinders is exhibited in a broken molar, the last in the series on the 

 left side of the jaw of the present cranium (See a figure of the grinding surface re- 

 stored of this tooth, fig. 2, PI. I.), and by another perfect molar, the last but one 

 on the right side of the upper jaw, which, though not belonging to the same in- 

 dividual as the skull here described, undoubtedly appertains to the same species. 



* Mem. de l'Acad. des Sciences de Paris, 1764, p. 568. 

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