38 OSTEOLOGY OF SCOMBROID FISHES 



The opisthotic separates the exoccipital from the pterotic to a vary- 

 ing extent, or not at all, but in no case is it interposed to so great a de- 

 gree as in most of the family Scombridoe. Usually the condition of this 

 bone does not differ from that of the Percoid fishes; i. e., the opisthotic 

 is chieflly on the lower surface of the skull covering the exoccipital- 

 pterotic suture, so that superficially, as viewed from below, the exoccip- 

 ital seems to be widely separated from the pterotic. As viewed from 

 above a very small portion of the opisthotic shows just behind the ex- 

 occipital-pterotic suture. Often there is a shallow notch at the end 

 of this suture in which the opisthotic appears, but to so small an extent 

 that the opisthotic can scarcely be said to separate the exoccipital from 

 the pterotic in any degree. The notch as viewed from above is a little 

 deeper in Oligoplites, Scomberoides, Alectis, Citula, Gnathanodon and 

 Megalaspis than in the other genera, and in Trachinotus it is very much 

 deeper, separating to a considerable extent the exoccipital from the 

 pterotic, approaching nearer the condition of the Scombridae than any 

 of the others do. 



In Oligoplites and Scomberoldes the lower posttemporal limb is at- 

 tached to the upper surface of the opisthotic, as it is in most of the 

 members of the family Scomberidae. In the others it varies from this 

 condition to that of having the lower limb attached more or less to the 

 posterior edge, as in the Percoids.* 



In Gnathanodon the anterior part of the pterotic incloses a very 

 large foramen, at least two-thirds as wide as the pterotic crest. This 

 is developed to a smaller degree in Decapterus and Trachurus; in all of 

 the others it is entirely absent. 



The basisphenoid is present and has a well developed process de- 

 scending to the parasphenoid, where it is rather broadly attached. 

 There is always a foramen just behind it, in front of the prootic suture, 

 and opening into the myodome. 



The alisphenoids are in all cases widely separated by the anterior 

 opening to the brain case. A pair of wings or ridges on the lower 

 surface of the frontals are continuous with the alisphenoid edges. They 

 converge forward except in Trachinotus, where they run straight and 

 nearly parallel to the prefrontals. 



The parietals are always well developed and form the middle por- 

 tion of the temporal crest. They are always widely separated by the 

 supraoccipital. 



*The distinction between posterior upper surface and posterior edge is not 

 great, and as it is somewhat variable, its importance is of questionable value. 

 Scomber is nearer to the Percoids in this respect than it is to the other genera of 

 its family, or to most of the Carangoids. 



