18 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



This latter tooth (Fig. 3, PL I. ; figs. 2 and 3, PL IV.) was found by itself, em- 

 bedded in the banks of the Rio Tercero, or Carcarana, near the Parana, at the 

 distance of a hundred and eighty miles from the locality where the head was dis- 

 covered. Fragments of a molar tooth of a Toxodon, apparently the seventh of the 

 left side, upper jaw, were also found at Bajada de St a Fe, in the province of Entre 

 Rios, distant forty miles from the mouth of the Rio Tercero. 



All the molar teeth are long and curved, and without fangs,* as in most of 

 the herbivorous species of the Rodent Order : in those, however, with curved 

 grinders, as the Aperea or Guinea-pig, and Cavia Patachonica, the concavity of the 

 upper grinders is directed outward, the fangs of the teeth of the opposite sides di- 

 verging as they ascend in the sockets ; but, in the Toxodon, the convexity of the 

 grinders is outward, and the fangs converge and almost meet at the middle line 

 of the palate, forming a series of arches, capable of overcoming immense resist- 

 ance from pressure. (See the upper view of the skull, Plate III., in which the 

 fractures expose to view a part of the series of these arched sockets.) 



Of the incisors, the two small ones (the sockets of which are indicated at 

 s s, PL III.) are situated in the middle of the front of the upper jaw, close to the 

 suture between the intermaxillaries, and the two large ones in immediate con- 

 tiguity with the small incisors, which they greatly exceed in size. The sockets 

 of the two large incisors (t t 9 PL III.) extend backwards, in an arched form, 

 preserving a uniform diameter, as far as the commencement of the alveoli of the 

 molar teeth : the curve which they describe is the segment of a circle ; the posi- 

 tion, form, and extent of the sockets of these incisors are the same as in those of 



the corresponding teeth of the Rodentia. 



The matrix, or secreting pulp of the large incisors, was lodged, as in the 

 Rodentia, in close proximity with the sockets of the anterior molars ; and we are 

 enabled to infer, from the form of the incisive sockets, notwithstanding the absence 

 of the teeth themselves, that the pulp was persistent, and that the growth of these 

 incisors, like those of the Rodentia, continued throughout life. 



This condition, joined with the form and curvature of the socket, implies a 

 continual wearing away of the crown of the tooth by attrition against opposing 

 incisors of a corresponding structure in the lower jaw : and as a corollary, it may 

 be inferred that the teeth in question had a partial coating of enamel, to produce a 

 cutting edge, and were, in fact, true dentes scalprarii. The number of incisors in 

 the upper jaw of Toxodon, is not without its parallel in the Rodent Order,^ the 

 genus Lepus being characterized by four, instead of two superior incisors, w 

 also present a similar relative size but have a different relative position, 



hich 

 the 



* True fangs exist only in teeth of temporary growth, they may be one or more in number, but always 

 diminish in size as they recede from the crown of the tooth, and are either solid, or with a very small canal. 



