GREGORY: FISH SKULLS 



141 



sphenotic both have the appearance of being endosteal bones, like the adjacent otic and 

 sphenoid elements, but on their dorsal surfaces (Fig. 33) they appear to be of the same 

 texture as the ectosteal parietals and frontals. 



On the dorso-lateral surface of the sphenotic is a well marked depression or fossa, 

 called by Ridewood (1904a, p. 61) and others the "lateral temporal groove or fossa," 



pfs.. 



sphot 



Tarpon 



Fig. 33. Tarpon atlanticus. Top view. 



In this relatively primitive isospondyl the exposed portion of the supraoccipital is small, the parietals are still in contact with 

 the frontals, the mesethmoid (dermethmoid) is small. 



The nasals, now well advanced beyond the ganoid stage, have become reduced in size and widely separated across the 

 mid-line. 



which gives origin to the dilatator operculi muscle. On the lateral surface of the otic 

 capsule is a deep fossa, bounded by the prootic, the pterotic and the opisthotic, which is 

 called by Ridewood (1904a, p. 62) the "subtemporal fossa." He notes that Sagemehl 

 (1884) found that a similar but larger fossa in the cyprinoid fishes serves for the lodgment 

 of the great muscles which, by pulling up the inferior pharyngeal bones (fifth cerato- 

 branchials), bring the teeth upon those bones forcibly against the callous pad that is 



