290 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
the constriction is filled by the element which Roth identifies with the 
pars mastoidea of the human skull. In the Toxodonta and Entelonychia 
and in some of Typotheria (e. g. Pachyrukhos ) this element contains a 
large cavity, which communicates with the cavity of the tympanic bulla 
by a small canal, but in most of the Santa Cruz members of this suborder 
is filled with cancellous bone, obviously a secondary condition. Roth 
gives no convincing reason for regarding this element as homologous 
with the human mastoid, which is a part of the periotic cartilage, while 
that so named in the Toxodontia is according to Roth the ossification of 
a membranous pouch, a difference which is not easy to harmonize. 
Rambaud and Renault (see Roth, p. 17) have shown that the human 
squamosal ossifies from three centres, for the squamous portion, the 
zygomatic process and the auditory portion (pars Serrialis ) respectively, 
and Roth figures skulls of Toxodon, in which these elements remain sep- 
arate in the adult stage. He does not make clear his conception of the 
relation between the pars Serrialis of the squamosal and the pars mas- 
toidea of the periotic. As a matter of fact, it is the pars Serrialis , or post- 
tympanic portion of the squamosal which is inflated, as clearly appears 
in those Typotheres in which the two are separate. Sinclair has figured 
(this volume, p. 71, fig. 14) a skull of Hegetotherium (A. M. N. H., No. 
9223) in which the element in question is divided by a transverse suture 
(fig. 14, mp.) into dorsal and ventral portions ; the former is the inflated 
pars Serrialis of the squamosal and the latter a thin, compressed plate 
ankylosed with the tympanic, which is the homologue of the similar plate 
in the skull of Nesodon which is called the mastoid or protuberancia pe- 
trosa. Attention should be called to the fact that my interpretation of 
these parts differs entirely from that of Sinclair, who has followed Roth. 
The occiput of the pig (Text-fig. 45) presents at least a very similar ap- 
pearance to that of the Toxodonta. Here the occiput proper, as in Neso- 
don , is wide at the base and gives off very long paroccipital processes. 
Above the foramen magnum is a very sudden and sharp constriction, the 
two exoccipitals being very much broader than the ventral portion of the 
supraoccipital. The posterior surface of the skull is completed on each 
side by a large area of the squamosal (pars Serrialis ) quite as in the 
Toxodonta, but with the notable difference that there is no cavity in this 
region. In the very young pig-skull the supra- and exoccipitals are rela- 
tively larger and the part taken by the squamosals in the formation of the 
