151 

 PROCEEDINGS OF NATURAL HISTORY SOCIETIES. 



GEOLOGICAL SOCIETY. 



April 19. — The Rev. W. Whewell, President, in the chair. — Mr. Owen read 

 a paper on the cranium of a newly discovered extinct animal, of gigantic pro- 

 portions, which he proved to be related by affinity to the Pachydermata and 

 herbivorous Cetacea ; he, however, further observes, that were its classification 

 determined by dentition, it would belong to the Rodentia. Mr. 0. names the 

 genus Toxodon. — " This cranium forms part of the series of fossils collected by 

 Mr. Darwin in South America. It was found in the Sarandis, a small tributary 

 of the Rio Negro, about 120 miles N.W. from Monte Video, and had been 

 imbedded in the whitish, argillaceous earth which forms the banks of that rivulet. 

 The subsoil of the whole of the surrounding country is granitic, and Mr. Darwin 

 considers the argillaceous covering to be an estuary deposit, accumulated by the 

 river now called the Plata, and at a period when the land was at a lower level 

 with reference to the ocean than it is at present. 



" The dimensions of this interesting fossil, the extreme length of the skull 

 being two feet four inches, and the extreme breadth one foot four inches, amply 

 attest that the species to which it belonged attained a magnitude comparable 

 only with some of the gigantic Pachyderms, or the extinct Megatherium. — From 

 the structure of the molar teeth and their continuous mode of growth, Mr. Owen 

 shewed that the Toxodon is referable to the Rodentia ; but that it differs from 

 the existing animals of that order in the number and relative position of the 

 incisors, and in the number and directions of the curvature of the molars. The 

 Toxodon again deviates from the true Rodentia, and resembles the Wombat, in the 

 form of the articular cavity of the lower jaw. It differs from the Rodentia and 

 resembles the Pachydermata in the relative position of the glenoid cavities and 

 zygomatic arches, and in many minor details. In the aspect of the plane of the 

 occipital region of the skull, in the form and position of the occipital condyles, 

 in the transverse extent of the frontal region of the skull, in the aspect of the 

 plane of the bony aperture of the nostrils, and in the thickness and texture of the 

 osseous parietes of the skull, the Toxodon differs from both the Rodentia and 

 Pachydermata, and manifests an affinity to the Cetaceous order. 



" From these instances of aberrant characters in the Toxodon, considered as a 

 gigantic Rodent, and which were described in detail, Mr. Owen pointed out, that 

 although the teeth, from their correspondence with many other important parts of 

 the animal structure, and from the facility of observing them, are highly important 

 and useful zoological characters, yet they are not, in all cases, sufficient alone to 

 determine the order to which a Mammifera belongs ; and that upon due consider- 

 ation it will appear, that dental characters must yield the precedence to those 



