68 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
lateral ethmoid on the upper wall, and turbinals on the medial and 
lateral walls. 
To place these bones in the terms of human anatomy: the four occipitalia fuse 
to form the single occipital of man; the six sphenoidalia similarly unite to form 
the single sphenoid, the alisphenoids forming the greater wings, the orbitosphenoids 
the lesser wings, while the ethmoidalia fuse to the ethmoid. 
In all bony vertebrates the cranial walls are completed dorsally 
by membrane bones, which in the lower fishes overly the tegmen cranii, 
while in the higher groups they replace it, the cartilage failing to develop 
Fic. 67.—Dorsal view of schematic skull, the chondrocranium dotted, cartilage bones 
with lines and dots. premax, premaxilla; pref, prefrontal; postfr, postfrontal; postor, 
postorbital; sqwamos, squamosal; guju, quadratojugal; qu, quadrate; inp, interparietal; 
exoc,"exoccipital; supratem, supratemporal; other names in full. 
in the roof. The number of these elements varies between wide limits, 
the following being the most constant. 
Beginning in front (fig. 67), there are, on either side of the median 
line a pair of nasal bones covering the olfactory region; a pair of 
frontals between the orbits; a pair of parietals at the level of the 
otic capsules, between which there is frequently a parietal foramen 
for the connexion of the parietal eye with the brain; and an inter- 
parietal, arising from paired centres, between the parietals and the 
supraoccipital. 
In the higher vertebrates (where the interparietal frequently fuses 
