SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 
137 
the adult, completely separating the postglenoid and post-tympanic pro- 
cesses and projecting freely below them, ending in a conspicuous, bluntly 
pointed mastoid process, of which there is no trace in the very young 
stage. Toward the mesial side it reaches and is closely applied to the 
tympanic bulla with which it may become fused. 
The tympanic, in the very young skull, is a large, oval, thin-walled and 
moderately inflated bulla, with long axis directed antero-posteriorly, in 
striking contrast to the small, scale-like tympanic of the Litopterna. In 
the adult skull the bulla has a different form, being more mamillate in 
shape, with the principal diameter dorso-ventral, somewhat as in the pig, 
but not in such an exaggerated degree, and throughout life it remains 
hollow and free from cancellous bone. The external auditory meatus is 
a very long tube, only the outer end of which is visible in the intact skull, 
and which passes upward, outward and backward to a very elevated ter- 
mination, which has nearly the same relative position as in Sus. 
This auditory region of the skull is, in Nesodon , quite different in its 
details from that seen in the small Typotheria of the Santa Cruz. Disre- 
garding the extraordinary specialization of this region in Pachyrukhos , 
the other and less extremely modified genera, such as Hegetotherium and 
Protypotherium , have a less extensively developed post-tympanic process, 
which occupies much less of the occipital surface and its cavity is filled 
with coarsely cancellous bone. The postglenoid and post-tympanic pro- 
cesses are more widely separated than in Nesodon and the space between 
them is filled by the tubular auditory meatus ; the mastoid is not visible 
externally and there is, of course, no mastoid process. 
The zygomatic process of the squamosal in Nesodon is moderately 
elongate, very deep dorso-ventrally, plate-like, thin and compressed later- 
ally ; its dorsal border is continuous with the lambdoidal crest and rises 
steeply upward and backward. The anterior end of the process is abruptly 
truncated and slightly convex, and forms little or none of the orbital 
boundary. The jugal is very large, especially in dorso-ventral diameter, 
but thin and laterally compressed ; the posterior border is excavated 
to receive the rounded anterior end of the zygomatic process and it 
extends far back beneath the latter, but not reaching to the glenoid 
cavity, as it does in the Santa Cruz Typotheria. Anteriorly, it rests 
upon the zygomatic process of the maxillary, extending upward to a con- 
tact with the lachrymal and excluding the maxillary from the rim of the 
