xm PHYLUM CHORDATA 439 



of the cranium. Roofing over the part of the cranial cavity the 

 walls and floor of which are formed by the sphenoid elements, 

 is a pair of investing bones, the parietals (Fig. 1093, pa.), and further 

 forward is another pair, thefrontals (fr.). The parietals are plate- 

 like bones, convex externally, concave internally, which articulate 

 with the supra-occipital behind by a transverse serrated lamb- 

 doidal suture. The right and left parietals articulate together 

 by means of a somewhat wavy suture, the sagittal ; in front a 

 transverse serrated suture, the coronal, connects them with the 

 f rentals. Between the supra-occipital and the parietals is a median 

 ossification or inter-parietal (int. pa.). The frontals are inti- 

 mately united along the middle line by means of the frontal 

 suture. Laterally their orbital plates form an important part 

 of the upper portion of the inner wall of the orbit ; above 

 this, over each orbit, is a curved, somewhat crescentic process, the 

 supra-orbital process. Between the alisphenoid below, the parietal 

 and frontal above, the frontal and orbito-sphenoid in front, and 

 the parietal behind, is a broad bone (sq.), the superior margin of 

 which is bevelled off : this is the squamosal. It is produced in 

 front into a strong zygomatic process, which curves outwards, then 

 downwards, and finally forwards, to unite with the jugal in the 

 formation of the zygomatic arch. Below the root of the process 

 is a hollow, the glenoid fossa. Behind the squamosal gives off a 

 slender process, the post-tympanic process (p. t. sq.), which becomes 

 applied to the outer surface of the periotic. 



Between the occipital and parietal bones, below and behind the 

 squamosal, are the tympanic and periotic bones. The tympanic 

 forms the bony part of the wall of the external auditory meatus ; 

 below it is dilated to form a process (ty. bul.) projecting on the 

 under surface of the skull the bulla tympani. The periotic is a 

 bone of irregular shape, its internal (petrous) portion (Fig. 1094, 

 peri) enclosing the parts of the membranous labyrinth of the 

 internal ear, and externally presenting two small openings the 

 fenestra ovalis and fenestra rotunda visible only when the tym- 

 panic is removed ; internally it bears a depression, the floccular 

 fossa, for the lodgment of the flocculus of the cerebellum. Part 

 of the periotic (mastoid portion) is seen on the exterior of the 

 skull between the tympanic and exoccipital. The periotic and 

 tympanic are not ankylosed together, and are loosely connected 

 with the surrounding bones, being held in position by the post- 

 tympanic processes of the squamosal. Between the tympanic and 

 periotic are two foramina of importance the stylomastoid, which 

 transmits the seventh cerebral nerve, and the Eustachian aperture, 

 at which the Eustachian tube opens. 



Roofing over the olfactory cavities are two flat bones the nasals 

 (nas.) each having on its inner surface a very thin hollow process, 

 the naso-turbinal. In front of the nasals are the premaxillae 



