GREGORY: FISH SKULLS 



315 



compared with the more primitive conditions in the ordinary mackerels, they also bring out 

 the relatively close relationships in all other characters even between the extremes of the 

 series, Scomber and Auxis, so that the separation into higher and lower "orders" seems to 

 be an extreme application of the "horizontal" system of classification. Both the illustra- 

 tions and the descriptions, however, support the author's conclusion that in a general way 

 the "scombroid" group (as he limits it) exhibits four successive grades of evolution, typified 

 respectively by the mackerels ("Scombridse"), seer-fishes ("Cybiidae"), tunnies ("Thun- 



meth 



A. Promethichtliys prometheus B. Lepidopas caudatus 



Fig. 194. A. Promethichthys prometheus. B. Lepidopus caudatus. Top views. After Starks. 



nidae") and bonitos ("Katsuwonidae"). It has been shown above that with regard to skull 

 structure this sequence involves the transformation of the relatively primitive and narrow 

 percoid skull top of Scomber into the specialized broad-snouted, broad-skulled type of 

 the tunnies. 



Gempylids. — This family, including the escolares, oilfishes, cutlass fishes and their 

 allies, begins with Ruvettus, which is quite near to Scomber. Starks (1911a) figures the 

 top view of the skull of Promethichthys prometheus (Fig. 194) of this family and we see at 

 once that it is a modification of the Scomber type, masked by general prolongation, marked 

 increase in size of the nasals and by the possession of a concave facet on the end of the 

 vomer for articulation with the short ascending branch of the premaxillae. 



