76 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
The hasioccii^ital (figs. 1 to 8, h.o.) has a concave posterior face provided on its 
ventral margin with a pair of stout accessory articular processes for articulation with 
a pair of precisely similar processes on the adjacent ventral edge of the centrum of the 
first vertebra (figs. 4 to 6). Anteriorly to these and on each of its lateral surfaces 
the basioccipital is furnished with a pair of short but stout processes for articulation 
with the doubly-facetted inner extremity of the inferior limb of the post-temporal (figs. 
1,4, and 6). Midway between the two series of articular processes the ventral surface 
of the basioccipital is perforated by a single median nutrient foramen (fig. 3). 
Dorsally the bone suturally articulates with the inferior margins of all three ex- 
occipital plates. 
The superior or cranial surface of the basioccipital is deeply excavated to form the 
floor, and to some extent the side walls also, of three fossae, one median, the “ cavum 
sinus imparis” of Weber, and two lateral, the “ foveae sacculi.” Horizontal 
ingrowths from the inner surfaces of the exoccipitals meet over the dorsal surface of 
the basioccipital and either fuze or remain separated by a median suture, thereby 
forming a thin bony roof for the three fossae, and, at the same time, a section of the 
cranial floor on which the medulla oblongata rests (figs. 6 to 8, e.o^). The floor and 
side walls of the cavum sinus imparis are furnished by the basioccipital alone, and its 
roof by the horizontal exoccipital plates ; the floor and inner walls of the foveae 
sacculi are also supplied by the basioccipital, their thin roof by the exoccipitals, but 
the thin outer walls are mainly formed by the inferior margins of the opisthotic plates 
of the exoccipitals (figs. 5, 7 to 9, c.s.i., f.s.). A comparison of these figures will 
sufficiently illustrate the mode of formation and mutual relations of the three fossae. 
In fig. 8 the roof of the skull has been removed in order to show the floor of the 
cranial cavity and jthe horizontal plates of the exoccipitals in situ (e.o.^). In fig. 9 
these plates have been partially removed so as to expose the cavum (c.s.i.) and foveae 
(f.s.), and their respective contents the sinus endolymphaticus (s.e.) and the two 
sacculi (s.). Fig. 5 represents a vertical longitudinal section through the cranium, 
bisecting the cavum sinus imparis (c.s.i.) ; while fig. 7 was drawn from a transverse 
section through the hinder part of the skull of Macrones aor, and shows the cranial 
cavity separated from each utricular recess by the intracranial prolongation of the 
neural plate of the exoccipital (e.o.'), and also the relative positions of the cavum and 
the two foveae (c.s.i., f.s.). From fig. 7 it will be apparent that the cavum and the 
foveae He paraliel to one another, but inasmuch as the latter are situated more towards 
the sides of the basioccipital they occupy a slightly lower level, and individually are, 
perhaps, somewhat smaller than the former. 
The foveae sacculi end blindly behind but in the dry skeleton the cavum sinus 
imparis communicates by means of a triangular and slightly constricted aperture with 
the neural canal over the hinder edge of the basioccipital (figs. 5, 6, and 8, c.s.i.). 
This aperture may be readily seen in a view of the posterior face of the skull, 
beneath the hinder margin of the roof of the cavum (figs. 6 and 8, c.s.i.). In the 
