36 OSTEOLOGY OP SCOMBROID FISHES 



For the accommodation of the more or less divided hyomandibular 

 head there are two depressions on the lower surface of the cranium on 

 each side. The anterior depression is nearly round, and occurs between 

 th prootic and the sphenotic. The posterior one is elliptical and is con- 

 fined to the pterotic. There is no indication of the depression on the 

 under side of the cranium, which in some genera of the Scombridae ex- 

 cludes the pterotic from the brain cavity. 



The supraoccipital crest is always well developed and rises in a 

 thin, high, blade-like crest, extending nearly the whole length of the 

 cranium, and reaching forward in all cases to the ethmoid. The crest 

 is formed anteriorly by the frontals. Above the frontals its base is 

 always expanded and thicker than above the supraoccipital. Poste- 

 riorly the crest extends behind the epiotics to above the occipital con- 

 dyle, where it is usually the highest. 



The temporal crest is always thin and high, and is formed by the 

 epiotic, the parietal and the frontal. In nearly all genera it reaches 

 the edge of the cranium a little in front of the middle of the orbital 

 cavity. In Citula and Oligoplites the crest ends more posteriorly than 

 in any of the others, and in the latter scarcely reaches the cranial edge 

 (see under pterotic crest). In Trachinotus the crest does not typically 

 reach the edge at all, but runs nearly straight forward to the front of 

 the frontal, where it ends considerably inward from the edge of the 

 cranium. This, however, is apparently not a stable character, as in 

 some of the young examples the crest on one or both sides is deflected 

 towards the edge and is then not different from the usual arrange- 

 ment. In Oligoplites the crest is produced to a slight spur on the 

 parietal and dips low on the epiotic and frontal. In Scomberoides the 

 crest is high throughout, as in the other forms. 



The pterotic crest in most of the genera reaches the edge of the 

 cranium just behind the anterior ending of the temporal crest. In 

 Decapterus, Trachurops and Trachunis it ends farther back than in 

 the others, or over the posterior end of the orbital cavity at a consid- 

 erable distance behind the temporal crest. Citula differs from the 

 others in having the pterotic crest united with the temporal at a con- 

 siderable distance behind the anterior end of the latter. In Oligoplites 

 the temporal and pterotic crests come together and sink into the level 

 of the frontal over the posterior edge of the orbital cavity much as they 

 do in Scomber. From this union only the slightest indication of a ridge 

 runs laterally to the cranial edge. 



Posteriorly the pterotic crest ends in a more or less well developed 

 secondary spine at the base of the backward projecting pterotic spine. 



