compakatn'e study of scombroid fishes. 305 



Skin and Scales. 



The skill is tliick ami well clevelu]3ed, aud its deeioer layer, the dermis, 

 is comijosed <>f sevenl liivei-s of oblique connective tissue, riinnins iu two 

 difterent dii^ectious, more or less ^lerpeudicular to each other, and altenmtin;^ 

 iu succession. The skin is more or less elastic, and extensile longitudinally, 

 but almost uonextensile transversely. In the Scombridio we count only U\<< 

 layers of connective tissue iu the skin, in the Cybiida) foiu' layers, and iu 

 the Plecostei about six layers. 



Scales of the so-called scombroid fishes are generally described as 



cycloid, but most of them are imperfectly ctenoid, as they aj'o toothed at the 



posterior margin, and liave no striation or only £\iut sti-iation at the surface. 



In the Scombridae scales are nearly cycloid, almost equal iu size and form, 



everywhere in the lx>dy, exce\3t those scales on the second dorsal, anal, aud 



the middle part of the caudal fin. Scales iu these parts are small and 



slei;der. In the Cybiid« scales are small, thin, and are often concealed under 



the skin or disapi^ear 6-om the most part of the body. The diÜcrcntiation of 



scales is more mai'ked than iu the ScombridiB ; those on the latei'al lino and 



those near the dorsal and ventral median lines are longitudinally elongated 



and densely crowded. Scales at the pectoral region are lai-ger and more or 



less differentiated to form the corselet. Iu the Plecostei the corselet is very 



•well develo^Ded. Scales iu it are very thick, and it is covered by a tough 



membi-ane, so that the pectoral region is doubly strengthened, pi-obably to 



protect the thick jwrtion of the cutaneous blood-vessels, peculiar and very 



inqxirtant to the Plecostei. The scales on each side of the base of the fjrst 



dorsal are x^retty large, rhombic, and are an-anged in several longitudinal 



rows. Small elongated scales are foimd on the external side of ths 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 Katsuwonidaä 



scales are not developed outside the coi^selet ; but in an old sti-ijaed Ixinito I 



found ninute scales scattered here and there outside the coreelet. These 



scales ai'e roundish and have a few concentric striae In the Scombnd.e aud 



Cybiida? small scales are found on the o^Ärcular bones ; but iu the Plecostei 



these lx»nes are entirely naked. Sc-ales on the cheeks ai'e much modified, 



