XXIII ACANTHOPTERYGII 675 
Division IL—SCOMBRIFORMES. 
No bony stay for the praeopercle. Spinous dorsal, if distinct, 
formed of short or feeble, slender spines. Epipleurals usually 
attached to the centra when ribs are sessile, or to the para- 
pophyses of the vertebrae, rarely to the ribs. Pectoral arch 
similar to that of the Perciformes, but pterygials sometimes more 
abbreviated. Ventral fins thoracic. Caudal fin, if well developed, 
with very numerous rays deeply forked at the base. 
Although bound by natural ties, the series of families that 
cluster round the Mackerel offer so many modifications of 
structure that it is almost impossible to draw up a diagnosis 
differentiating every one of its members from the Perciformes, 
with which they are closely connected, and from which they 
Fic. 414.—Caudal fin of Sarda orientalis. h.s, Hypural spine. 
hardly deserve to be separated. Even after removing many 
genera which have been united with them by my predecessors, 
and which will now be found scattered among various groups of 
the system, no good definition of the Scombriformes can be 
given. The Mackerel and Horse-Mackerel are taken as the 
pattern-forms around which more or less aberrant types are 
located, types yet not so aberrant as to be traced back to these 
familiar forms through a number of intermediate grades. As 
