66 REPORT—1846. 
moir was published in 1838. M. Quatrefages, in his ‘ Considérations sur les Carac- 
téres Zoologiques des Rongeurs,’ 4to, 1840, had corrected what he assumed to have 
been Prof. Owen's allocation of the Toxodon to the Rodent order. M. Quatrefages 
thought the so-called incisors of the Toxodon to be canines, affirming that their 
roots extended to the maxillary bones above the first molars; and he regards the 
Toxodon as having a nearer affinity to the Morse (Trichecus). Prof. Owen referred 
to his ‘Odontography,’ p. 411, for a refutation of Geoffroy St. Hilaire’s ideas that 
the scalpriform incisors of Rodents were canines; and alluded to the enamelled 
complex molars of the Toxodon as opposing M. Quatrefages’ idea of its relationship 
to Trichecus. 
An almost entire skull of the Mastodon Andium had been transmitted to the British 
Museum from the post-pleiocene beds of the Pampas of Buenos Ayres; its molar 
dentition was described, and a distinctive character of its tusks, in a strip of enamel 
two inches broad along their outer sides, was pointed out. 
Macrauchenia. To this genus of tridactyle Pachyderms, which is nearly allied 
to the Palzotherium by the structure of the feet, and to the Llamas (Auchenia) in 
the structure of the neck, Prof. Owen had referred a molar tooth of the lower jaw 
(No. 952, Mus. Coll. Chir), on account of its crown being composed of two upright 
half cylinders of equal height, as in the Paleotherium. A left ramus of the lower 
jaw, from the tertiary deposits of Buenos Ayres, has been received, containing six 
molar teeth, three true and three false, the last four showing the same form or pattern 
as the single fossil tooth from Patagonia, demonstrating the resemblance with the 
lower molar teeth of the Paleothere, except in this difference, viz. the absence of the 
third lobe in the last molar, by which the generic distinction of the South American 
Pachyderm was established; and an approach made to the rhinoceros. The Ma- 
crauchenia, to which Prof. Owen provisionally referred the fossil in question, differed 
however, like the Palzothere, from the Rhinoceros, in the greater exterior convexity 
and equal height of the two semi-cylindrical lobes of which the last premolar and 
the three true molars were composed; and it further differed from both Paleothere 
and Rhinoceros in the more simple form of the second and third premolars; the 
enamel is smooth and the dentine compact, and the coronal cement forms a thin 
layer. The longitudinal extent of the six molar teeth was nine inches. 
Nesodon, n. g. A genus allied to the preceding, but resembling the Anoplothe- 
rium, in the absence of any vacant interspace in the entire dental series, and in the 
equal height of canines and incisors, was established on the anterior part of the lower 
jaw and on two molar teeth of the upper jaw, discovered by Capt. Sulivan, R.N., in 
an arenaceous tertiary deposit on the coast of Patagonia. ‘The incisors, canines, and 
premolars of the lower jaw are not only in contact, but overlap each other like 
scales or tiles, and the molar teeth of both upper and lower jaw are characterized by 
islands of enamel, whence the generic name proposed. The incisors are six in num- 
ber. The characters described by Prof. Owen show some resemblance to Toxodon, 
in which also the large procumbent incisors overlap each other: the interval between 
Toxodon and Macrauchenia is evidently partly filled by the present remarkable genus. 
The extent of the sloping symphysis, the breadth of the lower jaw behind the sym- 
physis, and the depth of the ramus at the beginning of the first true molar, were 
severally two inches. The quadruped to which these fossils belonged must have 
been about the sizeof the Llama. Prof. Owen proposed to call the species Nesodon 
imbricatus, in allusion to the tile-like, overlapping arrangement of the anterior teeth. 
A second larger species of Nesodon was indicated by four or five detached teeth of 
the lower jaw from the same deposits. This species, of the size of the Zebra, it was 
proposed to call Nesodon Sulivani. 
As a check to the undue increase of so many large herbivorous species of the 
Megatherioid and Pachydermal orders, the great Machairodus, discovered in the 
caves of Brazil by Dr. Lund, who first supposed it a hyzena, was well-adapted. An 
almost entire skull had been, thence, transmitted to Paris, and had been referred 
by M. de Blainville to the genus Felis, who had published a figure of it. A speci- 
men of the same, or a closely allied species, displaying some characters not pre- 
served in the Parisian specimen, had been transmitted from the tertiary deposits 
of Buenos Ayres. Prof. Owen pointed out several differences establishing the, at 
least, subgeneric distinction of this remarkable carnivore, which equalled the 
Bengal Tiger in size, and had upper canine teeth of thrice the length. As Prof, 
