

16 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



A DESCRIPTION OF THE CRANIUM OF 



TOXODON PLATENSIS ; 



A gigantic extinct mammiferous animal, referrible to the Order Pachydermata, but with affinities 



to the Rodentia, Edentata, and Herbivorous Cetacea. 



The cranium, which is the subject of the present description, was found in the 

 Sarandis, a small stream entering the Rio Negro, and about 120 miles to the 

 N. W. of Monte Video : it had been originally embedded in a whitish argillaceous 

 earth, and was discovered lying in the bed of the rivulet, after a sudden flood had 

 washed down part of the bank. 



The zoological characters deducible from this cranium, forbid its association, 

 generically, with any known Mammiferous animal, and it must therefore be re- 

 ferred to an extinct genus, which I propose to call Toxodon* from the curved 

 or arched form of the teeth, as will afterwards be described. The specific 

 name, in the absence of other means of knowing the peculiarities of the animal 

 than those afforded by the skull, may be most conveniently taken from the dis- 

 trict (La Plata), in which its remains were first discovered. 



The dimensions of the cranium of the Toxodon Platensis amply attest that the 

 animal to which it belonged was of a magnitude attained by few terrestrial 

 quadrupeds, and only to be compared, in this respect, with the larger Pachy- 

 derms, or the extinct Megatherium. The length of the skull (of which a base 

 view of the natural size is given in Plate I.) is two feet four inches : the extreme 

 breadth one foot four inches. The other requisite admeasurements are given in 

 the table at the conclusion of this description. 



The general form of the skull, as seen from above, is pyriform ; but viewed 

 sideways, and without the lower jaw, it is semi-ovate ; it is depressed, elongate, of 



considerable breadth, including the span of the zygomatic arches, but becoming 



rather suddenly contracted anterior to them, the facial part thence growing nar- 

 rower to near the muzzle, which again slightly expands. 



Among the first peculiarities which strike the observer, is the aspect of the 

 plane of the occipital foramen, and of the occipital or posterior region of the 

 cranium, the latter of which inclines from below upwards and forwards at an 

 angle of 50° with the basal line of the skull. This slope of the back part of the 

 skull is one of the characteristics of the Dinotherium ; it is common to all the 

 Cetacea, and is met with in a slighter degree in many Rodentia, and in the great 

 Ant-eater and some others of the Edentate order. The corresponding aspect of 

 the foramen magnum presents nearly the opposite extreme to man in the occipital 



* To&v, arcus; odovs, dens. 



