the two lobes are oomparedp the lower is slightly longer than 

 the uppero There are four gills on each side and there is an 

 opening posterior to the fourtho The gill-rakers are not 

 developedc The left and right branchiostegal membranes are 

 joined in the Istiophoridaej, but in the Xiphiidae they are 

 separate and are both free from the isthmuso There are seven 

 branchiostegal rayso 



bo Internal morphology 



Figure 12„ —Diagrams of the structure of the rostrao a, Wahoo 

 bo Sailfishj Co Broadbill swordfishj, do Xiphiorhyn 

 chuSj , lo premaxillaryi, 2c articular (ethmoid?^;, 

 3o nasal, 4o frontalj, 5o maxillaryo 



Figure 13o=-Cranium of black tunao 



s 



io Osteology 



In the general view, the skeletons of these fishes differ 

 markedly from those of the Scombriformesj. which they are 

 supposed to be most closely related to, and furthermore the 

 differences between the Istiophoridae and the Xiphiidae are 

 also conspicuouso 



If we compare the skulls with those of the Scombri formes, 

 the frontals and prefrontals are notably wider,. This form has 

 probably arisen in order to support the long projecting upper 

 jawo The right and left frontal bones are closely joined 

 together except at their most anterior partso Their dorsal 

 surfaces are generally rather smooth without any conspicuous 

 projections^ The juncture of the frontals and the supraoccipi- 

 tals has a rounded projection on its dorsal surface forming 

 roughly a hemispherical shapeo There is absolutely no narrow 

 occipital crest developed in the supraoccipitalo Another 

 marked peculiarity is the conspicuous projection directed 

 posteriorly from the epiotic and the pterotico In the istio- 

 phorid fishes the nasal bones are broad and are broadly joined 

 to the frontal boneso Anteriorly they are joined to the pre= 

 maxillarieso In the Xiphiidae the nasal bones are very small 

 and are not directly concerned in the form of the upper jaw, as 

 they are in the istiophoridso The upper jaw is principally 

 formed from the premaxillaries and is almost solid with fila- 

 mentous structures like the septa in bambooo The maxillaries 

 are roughly rod-shaped and extend about one -half the total 

 length of the upper jaw, or less« The above-described form of 

 the upper jaw applies to the istiophoridso but in the Xiphiidae 

 the upper jaw is hollow v/ith occasional membrane=like partitions 

 like septao Thus the upper jaw is made up of four boneSo 



35 



