roMTAllVriVE STl'UY OF SCOXIimOIl) IXSIIES. 315 



of tlio voutriil jnedian lino of the skiiH, eonucctiug the otic region with the 

 etluuoiclal buues. At thr nutciior end it rests ou the; vomer, luid is very 

 firmly unite<l with it, aud at the ^xjsterior end it is embraced by the vontrid 

 sides of the Imsioccipital. Fur the most part the piraspheuoid is entirely 

 five fnim other lionos. At tlu^ nutt>rior part the bone is more or less flattened 

 with a dorsal uiotliau ridge, aud is imited to the prefi'ontals aud ethmoid 

 alxne. At the p)3terior end of the fi'ee portion, the paraspheuoid is rhombic 

 ill I'ross-section, lia\iug a ventral median k'(>el. Near the posterior end the 

 bone has two shoii lateral w iugs t:) uuite with the prootics. At the ]K)sterior 

 end, tlu! 1 oue bectnnes thiu, wide, aud is bent upwards at the lateral sides. 

 In the Scombridiü the pirasphenoid is very slender, aud in full grown forms 

 its posterior end nejirly closes the foramen l)etweeu the two ventral wings of 

 the basioccipital. A sharp venti'al median ridge is found underueath the otic 

 region. In the Cybiid.« tlie j)ar!ispheuoid is rather broad, forked at the lund 

 end, thus, leaving a smjill Uiirrow foramen, which communicates with tlie 

 niyodome. In the fishes of this family as well as of those of the Hcombridse, 

 the p)sterior part of th_i paraspheuoid is rather Hat. Tlie venti'al median 

 keel is scarce!}- developed, except iu Gi/mnosarda. In the TluiunidiC the 

 dorsal mediau keel of the parasphentud extends to a spot just Ix^low the 

 basisphenoid, i and is firmly united to the latter at the eud of the keel. 

 Generally there is a small veuti'al median hole near the posterior eud of the 

 paraspheiioid. It is remarkable tluit the paraspheuoid is broad in the 

 Plecostei, and is turned upwards at the lateral margin of the jxisterior part, 

 thus fonning a s^jecial tubular chaml)er, characteiistic to the Plecostei. The 

 cliamber lies below the myodome, aud is connected to it ^\ith .a narrow 

 longitudinal slit. Tluis the brain-cavity of the Plecostei is much sepirated 

 from the base of the cranium. The clmmljer is narrow and pointed anterior- 

 ly, but diverges behind, and ends with an elliptical or roimdish opening. In 

 the Katsuwouidie the doreal mediau keel of the jiaraspheuoid is not conspicuous 

 at the posterior end of the orbit. The parasphenoiilal cliamljer is Ijetter 

 develojied in this family than iu the Thunnida». In Auxis the paraspheuoid is 

 produced beliiud as a pair of long horns beyond the occiput. In the Katsuwo- 

 nidio the ventral mehian keel is liettw- develoi^etl than in the Thunnidsx). 



The supriu)ccipital is a mediau bone more or less elongated longitudinally, 



