PROFESSOR OWEN ON THE MEGATHERIUM. 
587 
Scelidothere the coronoid process is relatively shorter than in the Megathere, and 
the foramen near the fore-part of the base of that process is outside and below that 
base, not on the inside of it as in the Megatherium. The mastoid process is relatively 
shorter and the stylohyal pit is shallower ; the lacrymal bone is more distinct, and 
the foramen is larger in the Scelidothere than in the Mylodon or Megatherium. In 
all the essential characters of the lower jaw, as in the number, structure and kind of 
teeth, the extinct megatherioid quadrupeds more closely resemble each other, and the 
existing Sloths, than any other known existing or extinct animals. 
The number of the teeth, their length, equable breadth and thickness, and absence 
of fangs, their deeply excavated base, and the unlimited growth resulting from the per- 
sistent matrix, together with their composition of cement, dentine and vaso-dentine, 
without any true enamel, are characters common to the Megatherioids and Sloths. 
The form of the teeth differs in, and characterizes, each genus. It would seem that 
the Megalonyx, in the elliptical or subcylindrical shape of such of its teeth as are 
known, more closely resembled the existing Sloths than do the other Megatherioids. 
The Unau {Choloepus didactylus) resembles the Mylodon in the distance between the 
first and the second molars of the upper jaw; but the advanced molar assumes, in 
that existing Sloth, the form and proportions as well as the position of a canine, and 
the corresponding tooth of the lower jaw is similarly developed and separated from 
the other three teeth by a nearly equal interval. In the Ai {Bradypus tridactylus) 
the first molar in both jaws presents nearly the same proportionate size to the rest, as 
in the Megatherium, and is not separated from them by a wider interval than the 
rest, as it is in the Unau and Mylodon. In both species of existing Sloth, the last 
molar of the lower jaw is as simple in form as in the Megatherium : in the Mylodon 
and Scelidotherium it is larger and more complex in shape, the grinding surface 
being divided into two lobes by two oblique channels, which traverse longitudinally 
one the outer the other the inner side of the tooth : these grooves are more shallow 
in the Scelidothere than in the Mylodon, and the lobes of the tooth are more equal 
and more compressed. The grinding surface itself, in all the molars of both Mylo- 
don and Scelidothere, resembles that in the Sloths ; the two transverse ridges are 
developed only in the teeth of the Megatherium, which are longer in proportion to 
their thickness than in the Mylodon, Megalonyx, or Scelidothere. These modifica- 
tions, with the narrow palate, the close-set series of teeth, their great length, and the 
concomitant depth of the jaws, are features of resemblance to the maxillary and dental 
characters of the Elephant ; but the fundamental structure of the teeth, not only of 
the Megatherium, but of all its extinct congeners, is manifested in the present day 
exclusively by the restricted and diminutive family of the Bradypodidce. I conclude, 
therefore, the present section of this memoir by repeating the remark which I was 
led to make in a former memoir*, relative to the existing Sloths: — “These Mammals 
MDCCCLVI. 
♦ On the Mylodon rohuslus, 4to, p. 45. 
4 H 
