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582 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



other directly posterior ; the latter shorter than the former, but tliinner and 

 sharper-edged. The inner expanded surface of the bone between these two 

 crests is smooth and convex ; tiie opposite side presents two hollowed surfaces 

 (the posterior one especially excavated) divided by a ridge which runs out to 

 the point of tibio-tibular 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 cuter one rather larger than the inner, the two being 

 separated almost entirely by an intervening non-articular groove. The most 

 ])roiuberant 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 Jlbula, 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 portien 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 

 l>ropriety 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 ihe tibia below), 

 with very slender shaft, less than two inches long, with an enlarged narrowly 

 ovai liead, 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- 

 ti(m with the tibia opposite a similar and more extensive roughened space on 

 the latter bone. 



Pes. — There are eight true tarsal bones, besides a supplementary ossicle 

 which I do not recognize. The larg^ calcaneum reaches far back; the 



