6 MAMMALIA class v 



Between the alisphenoid and supraoccipital bones lies the plate-like 

 squamosal, bounded above by the parietal and often the back of the frontal, 

 and filling in the lateral wall of the brain case. From the lower part of the 

 bone arises a strong process [processus zijgomaticus) corresponding to the squamoso- 

 jugal arch of reptiles, which extends forward, carrying on its under surface a 

 concave articulation {fossa glenoidalis, gf) for the reception of the condyle of the 

 lower jaw. Behind this articular surface the posterior border is developed 

 into th.e, processus postglenoidalis (gp) projecting downward. The space between the 

 exoccipital and the squamosal is filled in by the bones of the auditory capsule. 

 In the cartilaginous mass forming the auditory capsule of mammals, ossification 

 starts from three centres, corresponding to the prootic, opisthotic and epiotic 

 of reptiles, but these all unite in the single dense periotic (Per), the inner part 

 of which encloses the auditory labyrinth and inner ear, while the outer portion 

 makes the mastoid process. In immature stages the tympanic bone consists 

 of a simple bony ring around the ear opening, which condition is normal to 

 the adult in marsupials and many insectivores ; but, as a rule, the outer rim 

 of the ring expands horizontally and forms the external passage or external 

 auditory meatus (earn) for the ear. The lower margin of this opening is 

 covered by a flattened bony plate which may be inflated, bladder-like, making 

 the tympanic bulla (Ty), which may be hollow or may be filled with bony 

 tissue. 



In many cases the auditory bulla is formed from a separate element (os bullae, 

 entotympanic), which may represent either a separated lamina of the periotic 

 or an ossification in the wall of the tubo-tympanal duct. From the front of 

 the tympanic occasionally projects downward and forward a pointed styloid 

 process. In the opening between the periotic and tympanic bones, that is, 

 the chamber bounded externally by the ear drum (tympanic membrane) and 

 internally by the fenestra oralis, lie the three minute ear bones, stirrup {stapies), 

 the anvil (incus), and hammer (malleus). The foot-plate of the stapes lies in 

 the fenestra ovalis of the periotic ; the handle (manubrium) of the malleus is 

 attached to the ear drum. In the anterior part of the tympanic bone is the 

 opening of the Eustachian tube, which admits air from the pharynx to this 

 middle-ear chamber ; the internal carotid artery enters the brain case by 

 various paths, sometimes passing through the tympanic chamber (many insec- 

 tivores and lemurs), sometimes piercing the petrotympanic mass (Tarsiiis, higher 

 primates), sometimes running along the inner side of the bulla (carnivores), 

 sometimes entering through the foramen lacerum medium (certain lemuroids). 

 The foramen lacerum posterius (fp) transmits the jugular vein and the ninth, 

 tenth, and eleventh cranial nerves. 



The appearance of the skull is largely dependent on the development of the 

 facial bones ; especiall}- the maxilla (mx). This bone usually carries along its 

 lower outer margin the teeth, and expands horizontally to make a large portion 

 of the hard palate. From the back of the maxilla rises a strong process which 

 is first directed outwardly, then backward, being connected by the jugal or 

 malar (Jii) bone to the zygomatic process of the squamosal. Thus the zygo- 

 matic arch is completed, and makes the lower border of the orbit and of the 

 temporal opening. In the anterior I'im of the orbit, between the frontal, 

 maxilla, and ethmoid, lies a small bone, the lachrymal (La), which is always 

 perforated by the lachrymal canal. The front of the snout is formed by the 

 two premaxillae {pmx), which usually carry some of the alveoli for teeth. The 



