TOXODONTS. 565 



THE TOXODONTS. 

 SUBORDER Toxodontia. 



The toxodonts may be defined as a group of more or less aberrant Ungulates 

 with tall-crowned and curved cheek-teeth, some or all of which grow from 

 persistent pulps, either permanently or during life; while at least one pair of 

 incisors in each jaw are rootless, and the third upper incisor, when present, is 

 placed in the line of the cheek-teeth. The vertebrae of the neck are short, with 

 flattened articular faces to the bodies, and the vertebral artery piercing the 

 transverse process in the ordinary manner. The wrist (when known) is of the 

 alternating type, while the ankle-joint is formed on the linear plan. The astragalus 

 is slightly grooved on its superior face, and inferiorly is like that of the Odd-toed 

 group, having no head for the navicular ; but the calcaneum, which is truncated 

 inferiorly, has a large articular surface for the fibula, as in the Even-toed section. 

 The number of toes varies from five to three ; the middle one being larger than 

 either of the others, and symmetrical in itself. The femur may or may not have a 

 third trochanter. The number of trunk-vertebras in the typical genus is twenty, 

 or intermediate between that of the Even and Odd-toed groups. In form the 

 cheek-teeth of the less specialised forms approximate to the Odd-toed plan of 

 structure ; and in all the genera the enamel is most developed on, or even confined 

 to, the outer sides of the cheek-teeth, although there may be vertical bands on some 

 of the other surfaces. More specialised in the structure of the feet and teeth than 

 the last group, phylogenetically the toxodonts may apparently be regarded as 

 related to the Odd-toed Ungulates, but as retaining certain features now common 

 to the Even-toed group, which have probably been inherited from common 

 ancestors. 



While, as aforesaid, the more generalised members of the suborder approximate 

 in the structure of their teeth and feet to the Odd-toed group, the specialised forms 

 assume a more or less Rodent-like type of dentition and limb-structure, which 

 must probably be regarded as an instance of parallelism. It may be added that, 

 from the retention of clavicles, these Rodent-like types must be derived from some 

 form less specialised than toxodon, in which those bones have disappeared. 



The toxodon was of the size of a large rhinoceros, and characterised by the 

 long and curved crowns of its molar teeth, which continued to grow throughout 

 life. There were only two pairs of incisor teeth and no tusks in the upper jaw, 

 although in the lower jaw the full number of these teeth were developed. The feet 

 were furnished with three toes. 



This genus occurs in the superficial deposits of Argentina, but is replaced in 

 the Miocene Tertiaries of Patagonia by certain allied forms known as nesodons, 

 which may be briefly defined as including toxodonts of medium or small size, in 

 which the limbs, and probably also the neck, were relatively longer and more 

 slender than in the typical genus ; while all the teeth, with the exception of the 

 second upper and third lower incisors developed roots in the adult state, and the 

 upper molars were of a type approaching that of the Odd-toed group, with a 

 distinct posterior valley, and the middle column forming a distinct lobe projecting 



