DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 271 
(1.) The jugal (or malar) bone is large, and reaches back so as to he over the 
cartilage of the glenoid cavity, thus helping to form the joint. 
(i.) The angular part of the lower jaw is greatly incurved, forming a remarkable 
hollow inside. 
In the endocraniui there are some very curious structures, that differ from what 
we find in the high forms of Mammalia, but which mostly agree with what is seen in 
the Sauropsida :— 
(a.) The nostrils are sub-terminal and give off large tongue-shaped cartilages to 
protect JACOBSON’S organs. 
(b.) The whole nasal labyrinth is small, especially in the young, not more than 
half as large as in an average Placental Mammal, and the cribriform plate is less 
depressed in front, and very limited in size, and is square in form. 
(c.) The orbitosphenoids do not form the presphenoid by meeting together below, 
but the presphenoid is as independent as the basisphenoid. 
(d.) There is no special optic foramen in the orbitosphenoid, but the optic nerve 
passes through the common sphenoidal fissure with the orbital nerves, and the Ist 
branch of the 5th; the 2nd branch, like the 3rd, has its own foramen rotundum, as im 
Man, and many other Eutheria. 
(c.) The next character is one of the most important: it is this, namely, that the 
orbitosphenoids are flush with the alisphenoids. The latter, which are extremely 
large, ossify a tract of the general cartilaginous side-wall of the embryonic skull— 
and not a free flap of cartilage, merely 
the highly developed “ chondrocranium,” 
continuous with the basal bar, as in the Eutheria. For in these latter the more 
bulky brain pushes out the lower part of the side-wall of the skull, leaving for some 
time a band of cartilage, which runs free from the alisphenoid, passing on from the 
orbitosphenoid up to the supraoccipital. In front, the orbitosphenoid is confluent 
with the ethmoid, so that but for the breach in the wall, made by the alisphenoid, 
there would be, even in Placental Mammals, a chondrocranium very similar to that 
of the skull. This breach does not take place anywhere among the Vertebrate types 
until we get above the Marsupials. The other character just mentioned, namely, the 
absence of a special optic foramen, is of similar import; there never is such a foramen 
until we are among the Placental Mammalia. 
(f) The alisphenoid helps to form the drum-cavity by developing behind its small 
external pterygoid process a shell-like growth, similar to the “anterior tympanic 
recess” of Carinate Birds. Thus, as the squamosal is a labyrinth of air-cavities, 
opening into the upper part of the drum-cavity, these and the tympanic recess in the 
alisphenoid greatly enlarge the space for air, Indeed, not only those parts, but the 
mastoid region of the auditory capsule, and the sides and top of the occipital arch, all 
became pneumatic—as in Crocodiles and Birds. 
(g.) The internal carotid arteries pierce the basisphenoid submesially—that part of 
the basis cranii is not perforated in the Eutheria,—-and the clinoid processes and the 
concavity for the pituitary body are but little developed. 
