COMPARATIVE STUDY OF SCOMBROID FISHES. 
317 
small part in tlie lateral wall of tlie brain-cavity, generally with two concavi¬ 
ties. In the Thunnidæ the sphenotic has a large concavity inside, and another 
large one outside. The latter forms the bottom of a deep pit on the ventral 
surface of the cranium. Tlie dorsal surface of the sphenotic, lying between 
the temporal and pterotic crests, is divided into two, by a process of the 
parietal, extending over these bones and joining the anterior, internal corner 
of the pterotic. In the Katsuwonidæ the splienotics are nearly like those of 
the Thunnidæ ; but they appear only *a little at the dorsal surface of the 
cranium between the two lateral crests, or they do not appear at all ( Auxis ). 
Moreover at their ventral surface, we find a depression at the posterior, internal 
corner. 
The basisphenoid is the smallest cranial bone, Y-shaped, median in 
position, and lies between the prootics and alisphenoids on the cranial floor. 
Tlie median vertical process is laterally compressed, and is united to the 
parasphenoid, thus dividing the mouth of the myodome into two. In the 
Scombridæ the median process is very long, narrow, but in the Cybiidæ and 
Plecostei it is rather broad. 
The epiotics form the dorsal posterior part of the periotic capsule, lying 
on both sides of the posterior part of the supraoccipital, and anterior to the 
exoccipital. They are joined posteriorly to the exoccipitals with a rather 
straight suture, externally to the pterotics, and anteriorly -with the parietals, 
and sometimes with the sphenotics. In the inner side of the cranial cavity, 
the epiotics are bounded by the supraoccipital, prootics, and exoccipitals, and 
sometimes by sphenotics as well. They have each a rough prominent epiotic 
process to unite with the flat dorsal process of the posttemporal. Tlie epiotic 
process is continuous to the temporal crest ; but in Scomber the process and 
the crest are separate. In the Scombridæ the epiotics are markedly prominent 
as the external posterior ridge of these bones is vertical as in the Serranidæ 
and Carangidæ ; but in the Cybiidæ and Plecostei the ridge gradually slopes 
downward and outward. In the Cybiidæ a deep groove or a canal is often 
found in the internal side of the epiotic to receive the anterior semicircular 
canal of the auditory organ. In Katsmvomis we find a triangular process in 
the internal side of the epiotic to separate the dorsal part of the anterior 
semicircular canal. 
