COMPARATIVE STUDY OF SCOMBROID FISHES. 
451 
in' the case of the genus Katsuwonus, the hypaxial blood-vessels are much smaller 
and shorter than the epaxial, and the plexus of blood vessels surrounding the dark 
red portion of the lateral muscle are united to the epaxial blood vessels only. In¬ 
deed the epaxial blood vessels of the Katsuwonidae seem to correspond to the entire 
cutaneous system of the Thunnidae, and the hypaxial vessels of the former seem 
to be sui generis. The posterior cardinal vein joins the right Cuvierian duct, and 
joining this cardinal vein is a small renal vein. The interhaemal rod of the 
vascular plexus attains the utmost development in Katsuwonus and Euthynnus. 
The rod is thicker than the diameter of the vertebral column, and is protected 
by the bony trellis formed by haemal processes of the column, from the 
enormous development of the inferior foramen. In the genus Auxis, however, 
the interhaemal rod of the vascular plexus is very thin, and the inferior 
foramen is formed in a few caudal vertebrae only, having no relation with the 
vascualr plexus. 
Kidneys much elongated. Ureters are nearly separate, running almost 
parallel to each other in the posterior slender portion of the kidneys. The 
spleen is smaller than that of tunnies and is situated at the anterior portion 
of the visceral cavity. 
In the hypaxal dark red portion of the lateral muscle, just below the 
series of intermuscular bones a large strong tendon from the second vertebra 
is sheathed with thin layers of some muscle segments, from the myotomes of 
the third and some succeeding vertebrae. Thus in each epaxial portion of the 
lateral muscle two concentric rings of muscle segments are found in the cross- 
section. This is quite characteristic of the Katsuwonidae. The axial side of the 
lateral muscle meets the axial skeleton in the epaxial as well as the hypaxial 
portion, and the dark red portion is more voluminous than in the Thunnidae. 
The vertebral column is very firm, light, and compact, allowing no lateral 
motion. In anterior precaudal vertebrae the neural canal is separated from 
the canal of the spinal ligament by a thin bony septum as in the Scombridae. 
Neural process of the first vertebra is more or less united to the centrum and 
the posterior dorsal zygapophyses are very well developed in the vertebra. 
Parapophyses are quite abortive. 
Dentigerous ossicles on the gill-arches are large and are arranged in one 
row only. Internal gill-rakers are well developed. 
