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BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



tinct, and, moreover, widely separated from each other by iuterveution of 

 a special ossicle, doubtless a sesamoid, in the axis of the foot immedi- 

 ately above the single terminal phalanx. 



The actual structure, both of the bones of the digits and of the horny 

 hoof, will be appreciated from a glance at the accompanying figure. 

 This is engraved of life size, front view, with the hoof withdrawn sufS- 

 ciently to display all the parts. The preparation is 

 from a young subject about three months old, in which 

 the proximal epiphyses of the phalanges are still evi- 

 dent. The pair of distinct proximal phalanges of nor- 

 mal characters, or nearly so, are seen to be succeeded 

 by nodular medial phalanges, which latter, as well as 

 the distal extremities of the proximal phalanges, are 

 widely separated by intervention of a special ossicle in 

 the axis of the foot. To these succeeds a single broad 

 and flattened terminal iDhalanx, obviously composed of 

 ithepairof distal phalanges anchylosed together. In 

 this specimen, the anchylosis is complete, even at so 

 early an age of the subject; its condition apparently 

 being not the result of progressive confluence of the 

 two bones, but of their original connation. 



The terminal phalanx is flattened and somewhat 

 scooped out on its posterior aspect, without trace of previous separation 

 into halves. In front, however, as shown by the figure, i*" presents a 

 central triangular elevation, apex downward, and base articulated with 

 the nodular ossicle above it, as if a wedge of bone had been thrust into 

 the axis of the limb between the primitive distal phalanges. This wedge- 

 shaped piece of bone is completely anchylosed with the present single 

 distal phalanx; and below its apex the edge of the bone is perfectly 

 continuous across the axis of the foot. 



The central nodular ossicle, which 1 have already mentioned as a sesa- 

 moid, articulates with all five of the bones of the foot. I cannot account 

 for its presence unless it be a displaced sesamoid, such as for example 

 that which is normal beneath the base of the distal phalanx of the horse, 

 and known to some as the " os subarticulatum". In the normal pig's 

 foot, there are several pairs of sesamoids beneath the phalangeal articu- 

 lations; and the bone in question may be regarded as a confluence of 

 the pair at the base of the distal phalanges, or of tw^o pairs at the bases 

 of the medial and distal phalanges respectively. The displacement of 

 these sesamoids brings the ossicle into i3osition in the axis of the foot 

 between instead of under the bones. Or, it may be that this ossicle is 

 a confluent pair of sesamoids from beneath the basis of the medial pha- 

 langes, and that the wedge-shaped piece of bone which appears upon 

 the front of the distal phalanx, consolidated therewith, represents sesa- 

 moids from beneath the distal phalanges. 



The horny hoof encases these bones as far as the distal extremities 



