80 
PROFESSORS T. W, BRIDGE AND A. C. HADDON 
The utriculus (%. 9, ut.) is a somewhat oblong sac lying in, bnt without com- 
pletely^ filling, the utricular recess (figs. 7 and 8, ut.r.), and sharply constricted off from 
the sacculus (fig. 9, 5.) although connected with the latter by a short but compara- 
tively narrow ductus sacculo-utricularis (d.s.u.). The three semicircular canals 
connected with each utriculus traverse the various periotic bones with which they are 
in relation for only a very limited portion of their extent, and for the most part lie 
free in the utricular recess. The arch of the posterior vertical canal [p.v.c.) traverses 
the supraoccipital and epiotic ; the horizontal canal (h.c.) occupies a groove on the 
inner surface of the epiotic, and for the remainder of its course a short canal in the 
pterotic ; the anterior vertical canal {a.v.c.) has its arch but slightly protected by a 
short canal in the prootic and a deep groove in the sphenotic. An oblique valve, 
similar to that described by Ramsay Wright (43) in the case of Amiurus catus, is 
also to be found in the narrow lumen of the ductus sacculo-utricularis of 2Iacrones. 
The sacculus (fig. 9, s.) is an elongated sac, expanded and somewhat pear-shaped 
behind, but tapering in front. Its narrow anterior extremity penetrates into a slight 
excavation in the hinder margin of the prootic, but the posterior portion, with its 
lagaena cochleae, occupy a bony chamber of a similar shape — the fovea sacculi — the 
formation of which has already been described. From the inner surface of the 
sacculus, before the latter enters its fovea, a thin-walled transverse duct is given off, 
which extends inwards, and in the median line of the floor of the cranial cavity joins 
the corresponding duct of the opposite side, and the two together constitute the 
transversely disposed ductus endolymphaticus {cl.e.). The ductus occupies the deep 
transverse groove in the cranial surface of the basioccipital (figs. 5 and 8, t.g.), 
while the ductus sacculo-utricularis traverses a slit-like groove leading from the trans- 
verse groove into each utricular recess {l.g.). At a point where the two canals unite 
the ductus endolymphaticus gives off a median, pear-shaped, and extremely thin- 
walled sinus endolymphaticus (the “sinus impar ” of Weber) which projects back- 
wards into, and almost fills, the cavum sinus imparis (fig. 9, s.e.). 
In our description of the mode in which the cavum sinus imparis and the fovem 
sacculi are formed we stated that the latter were closed behind, but that the former 
in the dry skeleton, communicated freely with the neural canal over the dorsal surface 
of the hinder part of the basioccipital. In the fresh specimen, on the contrary, the 
cavum widens out round the blind posterior extremities of the two fovese into two 
small, laterally-situated, and nearly spherical cavities (the “atria sinus imparis” of 
Weber) which lie on the upper surface of the posterior part of the basioccipital, 
and have partly bony and partly fibrous walls (fig. 9, at.). The roof of tlie atria 
is formed anteriorly by an extension backwards of the bony roof of the cavum sinus 
imparis, and posteriorly by a prolongation forwards of the fibrous posterior wall. The 
posterior wall of the two atria is formed by a thick mass of fibrous tissue (p.iv.) 
which rests on the dorsal surface of the centrum of the first vertebra, between the 
pit-like sockets for the reception of the two condylar processes of the stapedes, and 
