scott: toxodonta OF THE SANTA CRUZ beds. 
123 
with the degree of abrasion. In nearly all of the grinding teeth the enamel 
covers the external wall and is reflected over upon the anterior and pos- 
terior faces, but covers them only partially. On the inner side the band of 
enamel is narrower. The much abraded m-, shown in Text-figs. 17, 18, 
has the posterior half of the crown bare of enamel and in m^ and m^ 
there is none on the inner side. 
In the unworn condition, the essential pattern of the molars may be 
readily made out, but the homologies of some of the elements are far from 
satisfactory determination. From the external wall run two principal 
transverse crests, of which the anterior one forms the front wall of the 
crown and the posterior one arises about midway in the fore-and-aft length 
of the outer wall. These two crests are separated by a broad valley, which 
narrows upward along the inner face of the tooth, until the two crests 
meet and coalesce, closing the entrance to the valley. From the external 
wall, nearly half-way between the two transverse crests, arises a very 
prominent spur or crista, which divides the valley into two parts and is 
much thicker and more prominent than in the premolars. Another spur 
projects from the outer wall behind the posterior crest, which it joins, 
enclosing a deep pocket, partially open on the inner side. It might, how- 
ever, be more accurate to regard this second spur as the posterior crest 
and to say that the valley is divided into three parts by two parallel spurs. 
There is a difference between various teeth in this respect ; in some, 
the third of the transverse ridges appears to be the posterior crest, or 
metaloph, in others the fourth. Still a third spur arises much higher up 
on the inner face of the external wall and encloses a second posterior 
pocket. 
With the progress of abrasion, the appearance of the molars becomes 
greatly changed ; the cross-crests, which in early stages are nearly trans- 
verse, grow more and more oblique, the principal valley is closed on the 
inner side by the junction of the transverse crests, but long retains its 
Y-shape, owing to the prominence of the spur which divides it, while the 
two pockets are converted into lakes, of which the posterior, being larger 
and deeper, persists longer, but is eventually obliterated ; in very old 
teeth it even disappears in m-. In the fully adult animal, with moder- 
ately worn teeth, there is a certain resemblance to the molar pattern of the 
perissodactyls, especially of the rhinoceroses, but the unworn teeth show 
that this resemblance is entirely superficial and without taxonomic 
significance. 
