1 36 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
spongy bone, and, so far as I have observed, does not contain a sinus. 
This process is closely applied to a high thin plate of bone, which is the 
element usually called the mastoid portion of the periotic (the protuber- 
ancia petrosa of Roth) and is interposed between the postglenoid and the 
post-tympanic processes, an altogether exceptional position. The post-tym- 
panic portion, or pars Serrialis is a very remarkable and anomalous struc- 
ture ; it is extensively exposed on the posterior surface of the skull, of 
which it makes up a large part, larger than in the Typotheria or the pig. 
Its dorsal portion is inflated and contains a large, oval cavity, lying above 
the long, tubular auditory meatus ; at the antero-internal corner of this 
cavity is a small, tubular passage, which communicates with the cavity 
of the tympanic bulla through the inner end of the auditory meatus. 
The surface exposure of the post-tympanic varies considerably with the 
age of the animal ; in the very young skull it is relatively much less 
extensive and forms a smaller proportion of the posterior surface of the 
skull ; along the base, or ventral side, of the occiput it is completely over- 
lapped by the exoccipital and the paroccipital processes form the infero- 
external angles of the posterior surface, since, at this stage, there are no 
mastoid processes. In the adult skull, on the other hand, the post-tym- 
panic is much increased in relative size and, along the base of the occiput, 
projects well externally to the exoccipital ; the paroccipital processes are 
no longer at the infero-external angles of the occiput, the development 
of the mastoid processes displacing them from this position. 
The periotic, or petrosal, is relatively large and of the usual dense, 
shining texture. The internal surface has a very small and shallow fossa 
for the flocculus of the cerebellum and from the anterior end is given off a 
blunt, inwardly projecting and recurved hook. The internal auditory 
meatus is large and conspicuous and deep within it may be seen the divi- 
sion into two canals for the seventh and eighth cranial nerves respectively. 
On the external face the periotic is concave and is closely applied, but not 
ankylosed, to the large, hollow capsule of the post-tympanic, which it par- 
tially encloses. The mastoid portion (as it is here called, without dis- 
cussion of the general homologies of this element, the protuberancia petrosa 
of Roth) is, in the very young animal, very small and inconspicuous, for 
the postglenoid and post-tympanic processes are in contact distally and 
but very little of the mastoid is visible between them. As the skull grows, 
however, the mastoid portion increases rapidly, becoming a large plate in 
