40 OSTEOLOGY OP SCOMBROID PISHES 



forming a roof, and the ethmoid forms a septum between it and its 

 opposite fellow, though behind the ethmoid there is a communicating 

 passage, which in the fresh fish was probably filled with cartilage. The 

 olfactory foramen enters this chamber at its outer edge. Seriola and 

 Naucrates have only a trace of this chamber. It is particularly well 

 developed in Gnathanodon, but is lower on the prefrontal, and the 

 frontal forms no part of its roof, though the ethmoid forms a large 

 part of it. It is in this form much deeper than long, and extends 

 downward to the parasphenoid. The olfactory foramen enters the 

 posterior end of it. It is similar to Gnathanodon, though scarcely so 

 large, in Trachurus, Trachurops and Decapterus; in Megalaspis it is 

 considerably smaller. It is moderately developed in Citula and Alectis, 

 but the foramen is so far to its outer edge that it can be scarcely said 

 to enter it. In Chloroscombrus, Vomer and Selene it is not w r ell devel- 

 oped, and in Trachynotus, Caranx, Oligoplites and Scomberoides it is 

 not present at all. 



The vomer bears a keel on its upper surface, which is a continua- 

 tion of the ethmoid keel. At each side of the keel rests the ascending 

 premaxillary process. 



The parasphenoid is sharply keeled under the orbital cavity. At 

 the mouth of the myodome it sends a lateral wing up along the edge 

 of the prootic, but it is confined to that hone, never reaching to the 

 alisphenoid or sphenotic. Trachynotus differs from the ether genera 

 in having the parasphenoid expanded to two or three times its ordi- 

 nary width to form a broad, flat table, against which the greatly en- 

 larged superior pharyngeals lie. This expansion is reinforced above by 

 the prootic, which is developed outward to the edges of the para- 

 sphenoid, or a little beyond. Gnathanodon differs from the others in 

 having a protuberance on the parasphenoid under the prootics, which 

 contains a conical cavity with its apex pointing straight forward. This 

 is doubtless the homolog of a somewhat similar cavity in the Tunninas 

 division of the Scombridae, into which the posterior myodome foramen 

 enters. In Gnathanodon, however, it is farther forward, remote from 

 the foramen. 



Other cranial elements not mentioned do not depart to any not- 

 able extent from the majority of spiny-rayed fishes. 



The Lateral Head Bones. 



The lateral bones of the head do not differ materially from those 

 of the family Scombridas. The bones of the suspensorium set closely 

 together, and there are usually no openings between them. 



