582 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
other directly posterior; the latter shorter than the former, but thinner and 
sharper-edged. The inner expanded surface of the bone between these two 
crests is smooth and convex ; the opposite side presents two hollowed surfaces 
(the posterior one especially excavated) divided by a ridge which runs out to. 
the point of tibio-fibular articulation. The head of the bone on top is trian- 
gular, the anterior angle represented by the tuberosity for attachment of the 
extensor tendon. The great part of this space is occupied by the articular 
facets, of oval shape, the outer one rather larger than the inner, the two being 
separated almost entirely by an intervening non-articular groove. The most 
protuberant outer corner of the head of the bone bears a small cupped oval 
facet, entirely separate from the knee-joint, for the articulation of the fibula. 
The enlarged lower extremity has its articular face divided antero-posteriorly 
by a ridge into two principal facets; similarly, the inner malleolus is emargin- 
ate, presenting two (an anterior and posterior) bony prominences instead of 
a single directly lateral nodule of bone. 
The fibula, so important’a bone for purposes of classification among 
Rodents, is here perfect and entirely free from bony connection with the tibia. 
Nevertheless, in its lower fourth or third it is closely apposed to the tibia 
(that portion of the tibia which bends outward and backward) and firmly 
bound in such position, apparently capable of little, if any, independent move- 
ment. I have not taken occasion to examine for myself the state of the parts 
in Castor; but in that animal, in which the fibula is apparently described with 
propriety as “perfect” and “free”, bony union is said to sometimes occur 
in old individuals ; and I should not be surprised if such were the case with 
Haplodon also. But, in any event, such superimposed, or, so to speak, fortui- 
tous and progressive consolidation, is not to be confounded with the complete 
true anchylosis which is characteristic of the Myomorpha. The fibula is a 
perfectly straight bone (excepting a slight inclination toward the tibia below), 
with very slender shaft, less than two inches long, with an enlarged narrowly 
oval head, only a small part of which is articular, and with a well-formed, 
irregularly triangular malleolus, the inner aspect of which forms part of the 
ankle-joint. A small extent of the shaft is roughened for ligamentous connec- 
tion with the tibia opposite a similar and more extensive roughened space on 
the latter bone. 
Pes.—There are eight true ¢arsal bones, besides a supplementary ossicle 
which I do not recognize. The large caleaneum reaches far back; the 
