In speculating upon the nature o f the organized substances which the teeth
o f the Toxodon were destined to grind down, we must not o n ly take the structure
o f the tooth into consideration, but also the power o f perpetual renovation, which
wdl compensate for the defective quantity o f enamel in the grinders o f the
Toxodon, as compared with those o f the existing Ruminants and Pachyderms,
whose grinders, when once completed, receive no further addition o f dental substance
at their base. The Toxodon, in this character o f its dentition, participated
m the same advantages with the Capybara and the Megatherium,
Although we have been enabled to observe the structure o f the grinding teeth
o f the upper jaw o f the Toxodon in two examples only ; one, an insulated perfect
grinder corresponding to the sixth alveolus on the right side, and the other, a
portion o f the last grinder o f the left sid e remaining in the so ck et o f the head
previously described, y e t from the relations subsisting between socket and tooth
a very satisfactory opinion may he formed o f the structure o f those teeth which are
wanting, as well as o f their size. It thus appears, that the grinders o f the upper
jaw o f the Toxodon, are small and simple at the anterior part o f the jaw , and that
they increase (chiefly in antero-posterior extent) in size , a s well as in complexity
as they recede backwards in the jaw. In this respect, as weil as in size, the teeth,’
in the fragments o f the lower jaw ju s t described, ex a c tly correspond. There is ’
however, a slight difference in the lateral diameter o f the two sets o f grinders,’
those o f the lower jaw being narrower, as is usually the case, hut not in the same
degree as in the Horse or Ruminant. A greater difference obtains in the degree o f
curvature o f the two sets o f molars, those o f the lower jaw, e sp e c ia lly the posterior
grinders, being mucli le ss bent than the corresponding teeth o f the upper jaw.
I t is necessary to observe, also, that the conv ex ity o f the curve o f the inferior
grinders is directed outwards, as in the superior grinders ; while in the Guinea P ig
and Wombat, which have also curved grinders, the cohvexity is outwards in the
lower jaw, and inwards in the upper jaw.
Nevertheless, i f we take into consideration the close similarity which ex ists
between the teeth o f the upper jaw o f the Toxodon, and those o f this lower jaw in
more essentia l points, as in their persistent pulps, their characteristic structure and
form, the depth o f their sockets, and their relative sizes and complexity ; and when
we consider how the depth o f this lower jaw, and its narrowness in the transverse
direction, corresponds with the characteristic form o f the upper jaw o f the Toxodon,
and that to these resemblances is added an apparatus o f incisors adequate tJ
oppose the great dentes scalprarii o f the u pp e r jaw, the conclusion seems irresistiof
temporary groivth, is, ot necessity, dependent on the duration of these essential aids to nutrition; thus, a
sheep genendly wears down its grinders in twelve years, and its natural term of life is consequently limited’to
about that period.
ble, that the lower jaw, here described, must he referred, i f not to the same, at
lea st to a nearly allied spe c ie s o f Toxodon, as that to which the large cranium
belonged.
Further researches in South America, it is hoped, will lead, ere long, to the
completion o f our knowledge of the osteology o f this very remarkable and interestin
g genus o f extinc t mammiferous animals.
DESOMPTION OP PAETS OF TH E SKELETON OP
M A C R A U C H E N IA PA T A C H O N IC A ;
A large extinct Mammiferous Animal, referrible to tie Order P a clyd erma ta ; hut with affinities to
the Ruminantia, and especially to the CamelidcB.
I n the preceding pages the nature and affinities o f a large ex tinc t Mammal were
attempted to be determined from the cranium and teeth exc lusively ; we come
now to consider the remains o f a quadruped consisting o f bones o f the trunk and
extremities, without a fragment o f a tooth or o f the cranium to serve as a guide
to its position in the zoological scale.
I t may appear, even to anatomists and naturalists familiar with th e kind o f
evidence afforded b y a fossil fragment, that an opinion as to the relation o f the
present spe cie s to a particular family o f Ruminants, formed without a knowledge
o f the important organs o f manducation, must be vague and doubtful, but the
evidence about to be adduced, will be regarded, it is hoped, as more conclusive
than could have been d p r io ri expected.
Th e portions o f the ske leton o f the animal—which, in relation to the affinity
above alluded to, as well as from the length o f its neck, I propose to call Macr-
aMc/iiBM*— were discovered b y Mr. Darwin in an irregular bed o f sandy soil,
overlying a horizontal accumulation o f gravel on the south side o f P o r t S t. Julian :
and independently o f the circumstances under which they were found, their correspondence
with each other in size, colour, texture and general character prove
them to have belonged to one and the same individual.
T h e se remains include two cervical vertebree, seven lumbar vertebrae, all more
or le ss fractured; a portion o f the sacrum and ossa innomina ta ; fragments o f
the le ft scapula ; o f the le ft radius and ulna, and le ft fo re-fo o t; th e le ft femur
* UaKpos longus, avxtfv cervix: from the latter word Illiger derived Auchenia, his generic name of the
Llama, 'Vicugna, &c.