RODENTS TOXODONTIDiE. 241 



Squirrels. ]\Iy classification mil thus stand : 1. The Toxodontid^. 2. The Hysteicid^, Cavies, the 

 Porcupines, &c. 3. The Hyracid-=e. 4. The Leporid-'e, Hares. 5. The Scix'kid.e, Squirrels, &c. 

 G. The MuRiD-E, Mice. Mr. Waterhouse's arrangement is supposed to be in an ascending order, 

 the Hares being lowest, and the Squirrels most advanced in organisation. I pay no attention 

 to this : it is not advancement — but affinity which is my guide. Tlierefore I content myself 

 with placing those next the Marsupials, which appear most nearly allied to the members of that 

 order. 



The Rodents are found in every quarter of the world, but their metropolis is South America. 

 Next to it in preponderance of species comes North America. The two together counting nearly as 

 many species as are to be found in the whole of the rest of the globe. No species is found both 

 in South and North America ; no sj)ecies both in South America and Africa. None aboriginal in Aus- 

 tralia are foiuid anj'-n-here else. No species are foimd both in the Old World and the New. Some 

 authors make a few exceptions to this, but we shall presentlj' see that even these are doubtful. 

 Africa, Asia, and Europe, have species which are found in all three. In what I have above said 

 I of course do not take account of introduced species, house Rats and Mice, and such small deer. 



The exceptions above alluded to are five or six boreal species, regarding which it is doubtful 

 whether those found in the north of Europe and Asia are or are not the same as those inhabiting 

 the north of America. These doubtful species are the Polar Hare, the Beaver, the Musk Eat, 

 the Lemming, and Spermophilus Parryi. With these exceptions, which may be determined either 

 way, according to the ideas which each individual may entertain of what constitutes a species, there 

 are no two sjjecies common both to the Old World and the New. 



ToxoDONTiDyE. — When Speaking of the Nesobontid,^, I mentioned the circumstances imder 

 which the bones of it and of this genus were found by Mr. Darwin. They were discovered in 

 South America, near Bahia Blanca. Unfortunately all that were jjrocured were imperfect 

 portions of skulls, which, however, were so remarkable as greatly to add to the disappointment 

 that more complete specimens had not been found. D'Orbigny since then has added the descrip- 

 tion of the fore-arm of a second species of Toxodon. But that is, I believe, all that is j-et 

 known of the genus. 



It may well be supposed that with such scanty materials even the order to which these genera 

 belong is a matter of doubt, and it is only provisionally that they are placed here. That they 

 were animals of great size is apparent from tlie dimensions of the skull, that of Toxodon Platensis 

 being two feet four inches in length, and one foot four inches in breadth ; but whether they are 

 Rodents, Pachyderms, Ruminants or Sirenians, to all which they have been referred, or whether 

 they were aquatic or terrestrial, whether they had legs or fins, was all unknown «hen Professor 

 Owen described them, and is not much better knowni yet. 



The Toxodon Platensis had incisors like tlie hare ; a very small one behind a very large 

 one in each maxillary bone ; it had no canines, but a large vacant space between the incisors and 

 molars, as in Rodents ; the molars, seven on each side, diminishing in size as they advance to the 

 anterior part of each jaw, as in the Pachyderms, and also as in the Capybara, which, in this 

 respect, as well as in other peculiarities, shows afiinity to the Pachyderms. 



In Professor Owen's words, the dentition closely resembled the rodent tj-pe, but manifested 

 it on a gigantic scale, and tended to complete the chain of aflinities which link the Pachj-dermatous 

 with the Rodent and Cetaceous orders. The masticating and temporal muscles must have been 



