37-2 



ANATO:\rY OF VERTEBRATES. 



the base of the zygoma ; it is grooved below for the mandibular 

 joint, to which the malar contributes the outer part. The nasal 

 processes of the premaxillary do not extend so far back as the 

 nasals : the large antorbital vacuity, v, is reduced by the max- 

 illary zygomatic plate to a crescentic form. The zygomatic 

 expansion of the maxillary, 21, is deeply excavated on the inner 

 side; it forms, in the recent animal, a large bony capsule on 

 each side of the mouth, communicating therewith and lined 

 by the buccal membrane. A vertical sinus terminating below 

 in two small foramina, communicating with the orbit, di^ddes 

 the rhinencephalic from the prosencephalic fossa. A branch of 

 the lateral sinus leads from above the petrosal to between the 

 squamosal and tympanic externally. The olfactory cavity extends 

 backward beneath the rhinencephalic one, but not above it. The 

 ectopterygoid process joins the proj)er pterygoid, and, mth the 

 entopterygoid plate, completes a wide interpterygoid canal. The 

 base of the ectopterygoid is perforated by an ectocarotid foramen. 

 The squamosal is excluded from the cranial cavity by a fissure 

 V, hich widens as it descends between the squamosal and petro- 

 tympanic : a venous sinus occupies this fissure. A horizontal 

 septum divides an upper from a lower compartment of the anterior 

 half of the tymjDanic bulla. The sella turcica is shallow, and not 

 defined by clinoid processes ; the chiasmal platform is subqua- 

 drate, and leads to a fossa, perforated by the two large and ap- 

 proximated elliptical optic foramina ; a deep and narrow groove 

 extends from the optic fossa to the rhinencephalic compartment, 

 where it di^ddes to terminate at the orbito-ethmoidal foramina. 



The foramen rotundum and 

 foramen lacerum anterius 

 combine to form a large 

 subquadrate vacuity. The 

 cerebellar fossa on the upper 

 part of the petrosal is very 

 deep. The meatus internus 

 is extremely shallow, and 

 almost immediately divides 

 into the cochlear and vesti- 

 bular canals. - 



In the Porcupine (^?/5^n> 

 cristata)^ fig. 238, the occi- 

 pital region is nearly flat ; the paroccipitals descend only to the level 

 of the occipital condyles. The mastoid forms but a rough ridge. 

 The auditory bulhc are moderately developed ; the external meatus 



238 



Skull of the Porcupine. 



