GREGORY: FISH SKULLS 



419 



Dimensional Factors in the Mouth Region. — In order to attain a more graphic and precise 

 formulation of the chief dimensional factors in the varying lengths, positions and directions 

 of the mouth-parts, I have prepared the following definitions and the accompanying dia- 

 grams (Figs. 286-289). 



Leptolepis 



Fig. 288. Upturning of mouth correlated with depression of quadrate-articular fulcrum (Q). 



The prosthion (P) is the most anterior point of the snout (Fig. 286). 



The menton (M) is the most anterior point of the mandible. 



The pygidion is the mid-point of the narrowest part of the caudal peduncle. 



23 



