COMPARATIVE STUDY OF SCOMBEOID FISHES. 
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Skin and Scales. 
The skin is thick and well developed, and its deeper layer, the dermis, 
is composed of several layers of oblique connective tissue, running in two 
different directions, more or less perpendicular to each other, and alternating 
in succession. The skin is more or less elastic, and extensile longitudinally, 
but almost nonextensile transversely. In the Scombridæ we count only two 
layers of connective tissue in the skin, in the Cybiidæ four layers, and in 
the Plecostei about six layer's. 
Scales of the so-called scombroid fishes are generally described as 
cycloid, but most of them are imperfectly ctenoid, as they are toothed at the 
posterior margin, and have no striation or only faint striation at the surface. 
In the Scombridæ scales are nearly cycloid, almost equal in size and form, 
everywhere in the body, except those scales on the second dorsal, anal, and 
the middle part of the caudal fin. Scales in these parts are small and 
slender. In the Cybiidæ scales are small, thin, and are often concealed under 
the skin or disappear from the most part of the body. The differentiation of 
scales is more marked than in the Scombridæ ; those on the lateral line and 
those near the dorsal and ventral median lines are longitudinally elongated 
and densely crowded. Scales at the pectoral region are larger and more or 
less differentiated to form the corselet. In the Plecostei the corselet is very 
well developed. Scales in it are very thick, and it is covered by a tough 
membrane, so that the pectoral region is doubly strengthened, probably to 
protect the thick portion of the cutaneous blood-vessels, peculiar and very 
important to the Plecostei. The scales on each side of the base of the first 
dorsal are pretty large, rhombic, and are arranged in several longitudinal 
rows. Small elongated scales are found on the external side of the pectoral, 
and sometimes at the base of the ventrals and on the caudal. In the Plecostei 
scales round the pectorals are small and elongated. In the Katsuwonidæ 
scales are not developed outside the corselet ; but in an old striped bonito I 
found minute scales scattered here and there outside the corselet. These 
scales are roundish and have a few concentric striæ. In the Scombridæ and 
Cybiidæ small scales are found on the opercular bones ; but in the Plecostei 
these bones are entirely naked. Scales on the cheeks are much modified, 
