ON TRE ANATOMY OF FISHES. 
77 
fresh specimen, on the contrary, the aperture places the cavum in communication 
with two small laterally-situated cavities which lie on the dorsal surface of the 
hinder part of the basioccipital, behind the posterior edge of the bony roof of the 
cavum, and have partly osseous and partly thick fibrous walls — the “atria sinus 
imparis” of Weber. In figs. 4, 5, and 8, it will be noticed that the posterior edge of each 
exoccipital, beneath the origin of its horizontal plates that roof in the cavum sinus 
imparis and fovem sacculi, is deeply notched by a semicircular emargination ; this 
notch forms the anterior half of the external atrial aperture, or “ apertura externa 
atrii” of Weber, wdiich normally is completely closed by the spoon-shaped process of 
the stapes of its side. Anteriorly the cavum and fovese open into a deep transverse 
groove in the cranial surface of the basioccipital, situated just in front of the anterior 
margin of their bony roof (figs. 5 and 8, t.g.). The opening of the cavum is by 
means of a relatively small and somewhat triangular median aperture ; the fovese 
by two laterally situated, relatively large, and somewhat rounded orifices (fig. 5,f.s.) 
On each side the transverse groove communicates by a deep, but narrow, slit-like 
prolongation with the lateral recess of the cranial cavity {ut.r.) in which the 
utriculus and its semicircular canals are lodged (figs. 5 and 8, l.g.). The transversely 
disposed groove contains the ductus endolymphaticus, and its slit-like lateral 
prolongations, the ductus sacculo-utricularis of that side (fig. 9, d.e., d.s.u.). At the 
bottom of the transverse groove the dorsal surface of the basioccipital is raised into 
a sharp median ridge (fig. 8) which divides behind as it passes beneath the horizontal 
roofing plates of the exoccipitals into two divergent vertical ridges, and these by 
their union above with the exoccipital plates form the outer w^alls of the cavum sinus 
imparis and at the same time the inner walls of the two fovese sacculi (fig. 7). 
Anteriorly also the ridge divides, but into two horizontally diverging arms which 
together form the antei’ior rim of the transverse groove (fig. 8). In front of the 
transverse groove the two prootics meet in a median suture in the floor of the cranial 
cavity. In close proximity to this suture the posterior margin of each prootic is 
produced into a slender, backward-projecting spur of bone which forms the anterior 
lip of the groove for the ductus sacculo-utricularis and also overlies the entrance to 
an excavation in the substance of the prootic into which the contracted anterior 
extremity of the sacculus penetrates. 
As is the case in all Teleostei the inner wall of each auditory capsule is deficient 
and hence the latter appears as a deep lateral recess of the hinder part of the cranial 
cavity (figs. 5 and 8, ut.r). Externally each recess is bounded by the prootic, 
sphenotic, pterotic, and epiotic bones, and also by the posterior opisthotic plates of 
the exoccipital ; internally, each recess is in wide communication with the general 
cranial cavity. The entrance to each recess from the cranial cavity is bounded by 
two prominent ridges, one in front and the other behind, which slightly constrict the 
communication between the two cavities (fig. 5). The anterior ridge is developed 
from the inner surface of the prootic, and its free edge is directed obliquely backwards, 
