COMPARATIVE STUDY OF SCOMBROID FISHES. 
319 
the supraoccipital and pterotics. Each exoccipital diverges anteriorly, and 
extends also laterally in the Plecostei. In the Scombridæ and Gramm7t&rcynus 
there is an impression of the clavicular ligament on the bone. In the Cybiidæ 
and Plecostei the bone bears, on the dorsal side, an auxiliary intermuscular 
bone near the foramen magnum, and sometimes another auxiliary one in a 
little anterior and superior position. On the ventral side there is a large 
foramen for the exit of the vagus. In the Katsuw onidæ the exoccipitals are 
fused at the dorsal margin to form a prominent dorsal median crest, which 
lies just below the supraoccipital crest. The exoccipital crest is best developed 
in Auxis. On the inner side of the exoccipital, there are two or three grooves 
anterior to the origin of the spinal cord to receive a part of the auditory 
organ. 
The opistliotics are always found in the so-called scombroid fishes, and 
are generally seen from the dorsal as well as the ventral side of the 
skull ; but in the Scombridæ they do not appear at the dorsal side of the 
skull, except the articulating knob for the posttemporal. These bones lie on 
the exterior side of the exoccipitals, and are bounded by the prootics and 
pterotics on the anterior and exterior sides. They form a part of the posterior 
wall of the brain-case. They have a large rough process for the articulation 
of the hollow end of the lower process of the posttemporal on the dorsal side. 
The basioccipital is a bone with a concave occipital condyle behind, and 
a very deep concavity on the opposite side, lying just below the floor of the 
foramen magnum. The bone is bounded above by the exoccipitals, in front 
by the prootics, and ventrally by the parasphenoid. In the Scombridæ and 
Cybiidæ it is a narrow bone with nearly parallel horizontal sides in the lateral 
view. In the Plecostei the bone is produced ventrally below the horizon of 
the vertebral column. This is easily understood if you compare the side view 
of skeletons of different families in the accompanying plates. The expanded 
lateral wings of the basisphenoid overlap the posterior end of the parasphenoid 
from outside, protecting the parasphenoidal chamber. 
The nasals are more or less elongated flat bones, firmly joined to the 
anterior margin of the frontals, and the anterior end of these bones rests on 
the palatines. 
The preorbitals are also flat, elongated bones with an articulating surface 
