April 28, 1892] 



NATURE 



609 



Perissodactyle-Iike Ungulate, somewhat more specialized 

 as regards its dentition than Macrauchenia^ but exhibiting 

 strongly-marked Artiodactyle affinities in the ankle-joint. 



Still more remarkable are the generalized affinities dis- 

 played by the group known as the Toxodonts, of which 

 the first representative was also discovered during Darwin's 

 memorable voyage. These Ungulates cannot be included 

 in either the Perissodactyla or Artiodactyla, and, there- 

 fore, come nearer the original generalized Ungulate stock 

 than the animals already noticed. Toxodon, from the 

 Pleistocene of Argentina, was of the approximate size of 

 a Hippopotamus, and its osteology is tolerably well known. 

 It takes its name from the curvature of the molar teeth, 

 which approximate in structure to those of the Rhinoceros, 

 and, like the incisors, have ever-growing roots. The front 

 teeth are separated from the cheek-teeth by a consider- 

 able interval ; the upper dental series being reduced in 

 number by the loss of the outermost incisors and the 

 canines, and the lower by the disappearance of the first 

 premolars ; the lower canine is, moreover, rudimentary. 

 The feet conform to the Perissodactyle type in having 

 three toes, of nearly equal size, and also in the inter- 

 locking of the bones of the upper and lower rows of the 

 wrist- and ankle-joints. In the absence of a third tro- 

 chanter to the femur, and also in the articulation of the 

 fibula with the calcaneal bone of the ankle, as well as 

 in the structure of the palatal and tympanic regions of 

 the skull, Toxodon is, however, constructed on a decided 

 Artiodactyle type ; so that its characters are to a great 

 extent intermediate between the existing members of the 

 two groups. 



Going back to the earlier Tertiaries of Argentina and 

 Patagonia, a number of Ungulates allied to Toxodon, 

 but with much more generalized characters, have been 

 brought to light. The skulls from Patagonia brought 

 back by Darwin, and named Nesodon, also belong to this 

 same generalized group. In Nesodon there is the full 

 complement of 44 teeth ; and the same formula also 

 obtains in the recently described Protoxodon, in which 

 the feet are known to have been tridactylous in both 

 limbs, although retaining rudiments of the metacarpals 

 of the first and second digits, and being of a longer and 

 more slender type than in Toxodon. The allied animals 

 described as Acrot/ieriutn, some of which were about the 

 size of a pig, present a peculiarity totally unknown among 

 other Ungulates ; and, indeed, in any Eutherian Mammals 

 except some individuals of the small African long-eared 

 fox {0/ocyon). This peculiarity consists in the presence 

 of eight cheek-teeth on either side of each jaw ; the con- 

 stancy of this character being proved by its occurrence 

 in a considerable number of specimens. Whereas, how- 

 ever, in Otocyon the eight cheek-teeth are reckoned as 

 four premolars and four true molars, in Acrotherium 

 there are said to be five premolars and three true molars. 

 If this interpretation be correct, it is difficult to point out 

 a probable derivation for this most remarkable type of 

 dentition, since no other heterodont mammals arejdefinitely 

 known to have more than four premolars. 



If, however, the cheek-teeth really prove to be four 

 premolars and four true molars, there might be a possi- 

 bility of direct inheritance of the fourth molar of the 

 Marsupials, although even then there is the difficulty 

 that none of the Lower Eocene Ungulates of the United 

 States are known to have possessed more than three of 

 these teeth. And the probability accordingly suggests 

 itself that the additional tooth may be an acquired re- 

 dundancy. There are a number of other more or less 

 closely allied types which have received distinct generic 

 names, such as Colpodon and Adinotheriwii, but it is at 

 present somewhat difficult to realize all their distinctive 

 features and peculiarities. One genus, however, if the 

 specimen on which it was established is normal, is so 

 remarkable as to call for special notice ; and taken 



NO. I 174, VOL. 45] 



together with Acrotherium seems to show that these 

 South American Ungulates ran riot in the disregard of 

 all rules as to the number and arrangement of their 

 teeth. The genus in question is Trigodon, founded 

 upon the lower jaw of an animal about the size of a pig, 

 but evidently related in the structure of its cheek-teeth to 

 Toxodon. In this mandible the middle of the extremity 

 of the long and narrow symphysis is occupied by a single 

 cylindrical incisor tooth, flanked by a pair of larger in- 

 cisors, and these, again, by the still larger triangular 

 canines. If normal (and from Dr. Moreno's description 

 and figure it would seem to be so) this single median 

 incisor is totally unique in the whole mammalian class. 



A still more remarkable and puzzling group is typically 

 represented by the long-known Typotherium from some of 

 the Tertiaries of Argentina, which, while presenting many 

 dental characters connecting it with the Toxodonts, has 

 upper incisors resembling those of the Rodents ; with 

 most of which it also agrees in the presence of clavicles, 

 which are invariably absent in all true Un^'ulates. The 

 number of the teeth is similar to that obtaining in many 

 Rodents, with the exception that there are two pairs of 

 lower incisors. An allied type has, however, three pairs 

 of these teeth, thus departing further from the Rodent 

 type ; and the skull of both genera is constructed on the 

 Ungulate plan. All the teeth are rootless. From other 

 beds in Argentina we have the genus described as Hegeto- 

 therium, which, while having rootless teeth, differs from 

 Typotherium in possessing the whole typical series of 44, 

 without any marked interval between them. Here, then, 

 we have almost entirely lost the Rodent features which are 

 so marked in Typotherium, and thus revert nearer to a 

 normal Ungulate type ; it is unknown whether clavicles 

 were present. Still more generalized is an allied group 

 typified by Interathcrium, in which the dentition is 

 always complete, the anterior premolars have distinct 

 roots, and the incisors conical roots. This genus and 

 the allied Protypotheriutn thus appear to be connected 

 both with Typotherium and the Toxodonts ; the specific 

 name rodens applied to one of the species of Protypo- 

 theriiim apparently indicating the existence of Rodent- 

 like upper incisors. 



The existence of these intermediate forms renders it 

 exceedingly difficult to come to any satisfactory conclu- 

 sion as to whether Typotherium really has any genetic 

 affinity with the Rodents (among which it was placed by 

 the late Mr. Alston) ; for if there be such relationship it 

 would seem to imply the descent of all Rodents from a 

 form more or less closely allied to Interatherium—B. view 

 which can scarcely be maintained. 



That these Typotheroids were, however, in some 

 manner connected with the Toxodonts is tolerably clear ; 

 and there are nearly equally clear indications of a more 

 or less distant connection between the Toxodonts and 

 the Macrauchenias. The most probable explanation of 

 the latter relationship is that both groups took origin from 

 generalized Ungulates allied to those found in the Eocene of 

 the United States, and known as the Condylarthra, which 

 appear to have been the common ancestral stock of both 

 the Artiodactyle and Perissodactyle modifications of the 

 order. On this view the retention of characters common 

 to both the groups last-mentioned by the Toxodonts and 

 Macrauchenias is readily accounted for ; the Macrau- 

 chenias having acquired sufficiently well-marked Perisso- 

 dactyle characters to admit of their inclusion in that 

 group, while the Toxodonts cannot be placed in either 

 of the two existing divisions of typical Ungulates. Having 

 thus diverged at an early epoch (perhaps in the neigh- 

 bourhood of Central America) from the original general- 

 ized Ungulate stock, the ancestral Toxodonts and 

 Macrauchenias become the dominant forms in South 

 America, where they appear to have developed into 

 such numerous and unexpected modifications of struc- 



