420 



TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



The hornontal is assumed as the line joining the prosthion and the pygidion. 



The suspensorial point {H) lies midway between the centers of the anterior and posterior 

 facets for the hyomandibular. 



The quadrate pivot (0 is the mid-point of the articular hinge of the quadrate. 



The suspensorial line {HQ) joins the suspensorial point with the quadrate pivot. 



The suspensorial angle (a) is the inclination of the suspensorial line to the horizontal. 



The premaxillon (p), or premaxillary tip, is the most anterior point of the premaxillary 

 in norma lateralis (often p coincides with P, as in Fig. 286). 



The general line of the gape is usually the line pQ or PQ. 



The angle of the gape (0) is the inclination of the line of the gape either to the horizontal 

 or to a line parallel with the horizontal. 



The maxillary angle (0) of the closed mouth is the inclination of the alveolar border of 

 the upper jaw (projected in a parasagittal plane) to the horizontal axis of the body. 



The level of the quadrate-articular center (Q) is its distance (QQ') below the horizontal. 



The anteroposterior placement of the quadrate-articular center (Q) is its distance Q'H' in 

 front ( + ) or behind (— ) the projection of the mid-point {H) of the hyomandibular socket 

 upon the horizontal. 



The orbiton (0) is the most anterior point of the orbit. 



The level of the orbit is the height (00') of the orbiton above the horizontal. 



The height of the hyomandibular socket is the height {HH') of its mid-point (H) above 

 the horizontal. 



The effective rostral length {PO') is the distance from the prosthion or tip of the snout 

 to the projection (<?') of the orbiton (0) upon the horizontal. 



B 



Ffstularia serrata 



lylosurus acus 



Fig. 289. Contrasting conditions of Tylosurus and Fiitularia. (A) Short suspensorial line (HQ) combined with extreme 

 lengthening of PQ'. (B) Extreme lengthening of suspensorial line (HQ) combined with very short PQ'. 



■ In Figures 286-289 the points Q, H, are projected upon the horizontal as Q', H', 0'. 

 Taking the point H' as zero, a positive displacement of Q' will lengthen the gape, as in Fig. 

 2S5j4; a negative displacement of Q' will shorten the gape, as in Fig. 288. 



A lengthening of PQ' will decrease the angle of the gape (0), as in Fig. 289, while a 

 shortening of PQ' will increase the angle of the gape, as in Fig. 287C. 



