COMPARATIVE STUDY OF SCOMBROID FISHES. 
315 
of tlie ventral median line of the skull, connecting the otic region« with the 
ethmoidal bones. At the anterior end it rests on the vomer, and is very 
firmly united with it, and at the posterior end it is embraced by the ventral 
sides of the basioccipital. For the most part the parasphenoid is entirely 
free from other bones. At the anterior part the bone is more or less flattened 
with a dorsal median ridge, and is united to the prefrontals and ethmoid 
alx>ve. At the posterior end of the free portion, the parasphenoid is rhombic 
in cross-section, having a ventral median keel. Near the posterior end the 
bone has Wo short lateral wings to unite with the prootics. At the posterior 
end, the bone becomes thin, wide, and is bent upwards at the lateral sides. 
In the Scombridæ the parasphenoid is very slender, and in full grown forms 
its posterior end nearly closes the foramen between the two ventral wings of 
the basioccipital. A sharp ventral median ridge is found underneath the otic 
region. In the Cybiidæ the parasphenoid is rather broad, forked at the hind 
end, thus leaving a small narrow foramen, which communicates with the 
myodome. In the fishes of this family as well as of those of the Scombridæ, 
the posterior part of the parasphenoid is rather flat. The ventral median 
keel is scarcely developed, except in Gymnosarda. In the Thunnidæ the 
dorsal median keel of the parasphenoid extends to a spot just below the 
basisphenoid, i and is firmly united to the latter at the end of the keel. 
Generally there is a small ventral median hole near the posterior end of the 
parasphenoid. It is remarkable that the parasphenoid is broad in the 
Plecostei, and is turned upwards at the lateral margin of the posterior part, 
thus forming a special tubular chamber, characteristic to the Plecostei. The 
chamber lies below the myodome, and is connected to it with a narrow 
longitudinal slit. Thus the brain-cavity of the Plecostei is much separated 
from the base of the cranium. The chamber is narrow and pointed anterior¬ 
ly, but diverges behind, and ends with an elliptical or roundish opening. In 
the Katsuwonidæ the dorsal median keel of the parasphenoid is not conspicuous 
at the posterior end of the orbit. The parasphenoidal chamber is better 
developed in this family than in the Thunnidæ. In Auxis the parasphenoid is 
produced behind as a pair of long horns beyond the occiput. In the Katsuwo¬ 
nidæ the ventral mehian keel is bettor developed than in the Thunnidæ. 
The supraoccipital is a median bone more or less elongated longitudinally, 
