xra PHYLUM CHORDATA 309 



the former bending forwards and upwards to form with the post- 

 orbital the superior temporal arch. At the anterior extremity of 

 the snout is a median bone formed by the coalescence of the two 

 premaxillcB (p. mx) ; this bears the four anterior teeth of each 

 side. On each side behind the premaxilla is the maxilla (max), 

 consisting of two portions, an alveolar bearing all the rest of the 

 teeth, and a palatine extending inwards on the roof of the mouth, 

 together with an ascending process articulating with the nasal and 

 pre-frontal above. Articulating behind with each maxilla is a 

 jugal (ju), which forms the posterior half of the ventral boundary 

 of the orbit. The quadrate (qu) articulates movably with the 

 parotic process, and bears at its distal end the articular surface for 

 the mandible. 



In the anterior portion of the roof of the mouth, articulating 

 in front with the premaxiUse and maxillae, are the vomers (vom). 

 Behind and embracing them posteriorly are the flat palatines 

 (pal). The elongated pterygoids (ptg) articulate in front with the 

 posterior extremities of the palatines : behind each articulates 

 with the corresponding basi-pterygoid process (has. ptg) of the basi- 

 sphenoid, and sends back a process which becomes applied to the 

 inner face of the quadrate. A stout bone which extends between 

 the maxilla externally and the pterygoid internally is termed the 

 transverse bone or ecto-pterygoid (trans). Extending nearly verti- 

 cally downwards from the pro-otic to the pterygoid is a slender 

 rod of bone, the epi-pterygoid (col). 



The columella is a small rod partly composed of cartilage and 

 partly of bone, the outer end of which is fixed into the inner surface 

 of the tympanic membrane, while the inner is attached to a small 

 aperture, thefenestra ovalis, in the outer wall of the auditory capsule 

 between the pro-otic and the opisthotic. 



Certain depressions or fossae and apertures or foramina are to be 

 observed in the skull. The foramen magnum, the parietal foramen, 

 and the orbits have been already mentioned. The posterior tem- 

 poral fossa is situated on either side of and above the foramen 

 magnum, bounded above and externally by the roofing bones, and 

 on the inner side by the bones of the occipital region. The inferior 

 temporal fossa is bounded internally by the pterygoid, and is sepa- 

 rated from the palatine foramen by the transverse bone. The lateral 

 temporal fossa is the wide space in the side wall of the skull behind 

 the orbit ; the bony bar which limits it above is the superior temporal 

 arch ; a bony inferior temporal or quadratojugal arch is here absent. 

 The tympano-eustachian fossa, situated in the auditory region, is 

 bounded by the bones of that region together with the quadrate. 

 The posterior or internal naves are bounded posteriorly by the 

 palatines. The anterior or external nasal aperture is situated at the 

 anterior extremity of the skull bounded by the nasals and premaxillse. 



Each ramus of the mandible consists of six bony elements in 



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