GREGORY: FISH SKULLS 



221 



Synentognathi (Needle-fish, Flying-fish, etc.) 



The skull of the needle-fish (Tylosurus) (Figs. 99, 100) is specialized in many features 

 relating to the excessive elongation of the pointed jaws and of the body, the latter being 

 adapted for leaping from the water. The bill, or upper jaw, is composed largely of the 

 prolonged premaxillae. This is braced posteriorly by the narrow maxillae (which barely, 

 if at all, reach the angle of the gape) by the forwardly-produced palatines, the thin vomer, 

 the long parasphenoid, the forwardly-produced naso-proethmoids (Starks, 1926a, p. 208), 

 and by the delicate, minute, disc-like mesethmoid. The cranial table is flat and elongated, 



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B 





Tylosurus acus ^ 



Fic. 99. A. Tylosurus marinus. 



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T. marinus 



B. Cross-section along a line XX'. 



the frontals being the dominant elements. The orbits are fairly large. The minute supra- 

 occipital meets the frontals and the epiotics are crowded in toward the mid-line. As seen 

 from above the skull ends posteriorly In a pair of divergent horns, formed mostly from the 

 posttemporals and pterotics. The braincase is a rather narrow stiff trough. 



The elongate mandible is supported posteriorly by a forwardly-produced suspensorium, 

 which bears stiffening ridges on the quadrate and metapterygoid. The upper limb of the 

 preopercular is inclined slightly forward, the lower limb is horizontal. The roof of the 

 mouth is stiflfened by the parasphenoid. As is well known, the lower pharyngeals are 

 coalesced and support a triangular dentigerous surface, the teeth of which can be opposed 

 to those on the upper pharyngeals. Thus stiffness and lightness seem to be the main adap- 

 tional features in this long-beaked skull. 



The skull of the flying-fish Halocypselus evolans (Fig. 101) somewhat recalls that of 

 Fundulus in the relatively small, upturned mouth, triangular pharyngeal plate, smooth, 

 rounded operculars, disc-like mesethmoid. But it is obviously nearer phylogenetically to 

 the skull of Tylosurus, although contrasting with the latter in its small jaws without a beak. 

 From the excellent studies of Nichols and Breder (1928, p. 439) it even appears probable 

 that the short-jawed Halocypselus skull has been derived from one more like that of Tylo- 

 surus by the rapid shortening and finally by the complete loss of the beak. Possibly 

 reminiscent of a long-jawed stage are the bracing of the mandible by lateral ridges on the 

 quadrate and metapterygoid, the prolongation of the postero-superior border of the man- 

 dible, the buttressing of the small upper jaw by the enlarged lateral ethmoids. 



The characters of the back of the skull in both Tylosurus and Halocypselus have been 



