310 
A MEMOIR UPON THE GENUS 
portion anteriorly. The ectal fixcet may articulate posteriorly with the tibia, 
although this is a marked exception to the rule, as generally the astragalus extends 
too far\ehind for such an articulation, (tarsus No. 10,282 shows a slight exposure 
of the ectal facet of the calcanemu for the tibia.) A fibido-calcaneum fiicet may 
be present, although in some specimens it is not well shown. 
Three out of five calcauea of this species in the collection exhibit a 
fibular facet. The sustentacnlar facet is very large. It is oval in form and 
slightly bent toward the cuboid side. This facet is widely separated from the in- 
ferior. ^ The size of the sustentacular facet may vary considerably, being in some 
cases short and broad, or it may be more elongate than usual. The inferior facet 
of the calcaneum is well marked. It is rather elongate and narrow. It extends 
about half way across the anterior border of the bone and is not separated by a 
ridge from the cuboid fiicet. The position of the inferior facet is very ohliipie. 
The calcaneo-cuboid fiicet is large and deeply concave from above downward. 
Its middle portion transversely is nearly plane. This facet is bordered externally 
by a rounded margin and internally by a straight border which terminates above 
in a prominent process. A longitudinal fossa separates the sustentacnlar from the 
ectal facet of the calcaneum and corresponds with the depression upon the astra- 
galus, whereby along, narrow opening is produced when the two bones are in juxta- 
position. This fossa is very large in Titanotherium. 
Astragahis. — The astragalus is relatively short and broad ; its trochlear sur- 
face is broad and low. The external trocldear is bordered by a prominent, incurved, 
flat process, Avhich is a continuation of its anterior margin. Posterior to the inser- 
tion of this process upon the internal side of the trochlear there is a deep fi)ssa. 
The antero-posterior diameter of the twm troehlem are generally equal, l)ut in some 
specimens the external may be excavated behind to allow of the contact lietween 
the tibia and calcaneum. The facets of the inferior surface of the astragalus are 
separated, the inferior especially being widely isolated from the sustentacular. 
The ectal facet is very large and deep, and it sends a narrow portion anter- 
iorly. The antero-posterior diameter of this facet is much greater than the 
transverse. The sustentacular facet is large and extends to the anterior 
face of the astragalus ; it there abuts against tlie cuboid facet, being separ- 
ated from the latter by a slight ridge. The inferior facet of the astragalus is 
characteristic, as it is small and widely separated from the sustentacu- 
lum. The inferior facet only takes up about one-third of the whole culjoid 
border of the bone. We shall see later that in the forms related to P. palitdosiis 
the inferior facet is much larger, as is the case with Diplacodon, where the susten- 
taculum is reduced and the inferior fiicet larger than in P. paludosus. The section 
of the anterior face of the astragalus is triangular, the apex of the triangle being 
formed by a narrowing of the bone at the junction of the snstentacular witli the 
cnboidal facet. The latter fiicet forms an angle with the navicular face gi’eater 
than 90 degrees. In PdyracJiyiis and the smaller species of Palceosyops the cuboid 
