242 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



surface, which must fit loosely into a socket in the cartilaginous portion of the olfactory 

 capsule. 



The relations of the premaxills and maxillae to the ligaments connecting them with 

 the skull are shown in Figure 116, after Allis. 



The lower end of the maxilla is tied to the lateral surface of the mandible by a loose 

 fold of skin and connective tissue. The tendon of the superficial portion of the adductor 

 mandibulse (ad. I) is inserted on the medial surface of the shaft of the maxilla, behind its 

 articular process and near the fork of the palatine. Contraction of this muscle would tend 

 to pull the maxilla and premaxilla backward and slightly downward and thus help the other 

 adductors to close the mouth (Vetter, 1878, pp. 495, 497). In Roccus lineatus, a typical 

 percoid, depression of the mandible by the action of the paired geniohyoideus muscles 

 merely tends to rotate the premaxillse slightly outward and to protrude them slightly. 



Marked protrusion of the premaxillae can only take place under the following condi- 

 tions: (1) the gape must be small; (2) the fold of skin attaching the maxilla to the mandible 

 must be moved far in front of the fulcrum in order to exert a downward pull on the pre- 

 maxillae; (3) the quadrate-articular fulcrum must be brought much below the level of the 

 closed mouth; (4) at the same time the ascending process of the premaxillae must be greatly 

 prolonged. Under these circumstances a long downward movement of the mandible under 

 the pull of the geniohyoideus muscles will draw the lower ends of the maxillse sharply down- 

 ward, while the premaxillae are pulled downward and forward, the upper end of the ascend- 

 ing processes remaining on top of the vomer. 



Stages preceding the protrusile condition of the jaw may be summarized as follows: 

 (1) the premaxillae and maxillae together form the dentigerous outer border of the upper 

 jaw, both being fixed to the primary upper jaw. This stage is well illustrated in the Palae- 

 ozoic ganoids and in the living Polypterus. (2) The posterior end of the maxilla protrudes 

 laterally, but little or no movement is possible; e.g., Amia. (3) The proximal end of the 

 maxilla acquires a movable articulation with the vomer, the small premaxilla is closely 

 fastened to the maxilla and moves with it, as in the tarpon. (4) The premaxilla lengthens 

 its alveolar process and finally excludes the maxilla from the gape, and it also begins to send 

 out another ascending process growing upward above the vomer. An early stage in this 

 development may be seen in Mugil. (5) The proximal part of the premaxilla becomes 

 differentiated into ascending and articular processes. By this time the maxilla has com- 

 pletely lost its function as a dentigerous element and has become merely a lever for the 

 partial eversion and closure of the mouth. Doctor Allis' view that the ascending process 

 of the teleost premaxilla represents a formerly independent element which has become fused 

 with the main part of that bone is considered at some length on page 96 above. 



Mechanism oj the Neurocranium. — In Micropterus (Fig. 113) the'skull is of very primi- 

 tive percoid type. In Lates niloticus (Fig. 114) of the family Centropomidae specialized 

 features are the forward position of the eye, the sharp snout, the gently concave forehead 

 leading to the high back, and especially the prominent spikes on the angle of the preoper- 

 cular. 



The neurocranium of Luciolates (Fig. 117) is perhaps even better adapted than that 

 of the isospondyl tarpon (Fig. 32) to resist the stresses generated within it by the thrusts 

 coming from its own locomotor, masticatory and respiratory apparatus. It is more compact 

 than that of the tarpon in that the anterior prolongations of the spinal muscles in the oc- 



