182 LOWER VERTEBRATES. 



with small spines along their upper and lower margins. The dorsal fin is deeply 

 divided, or double, and the spinous portion much the longer. The ventral fins are 

 provided each with a spine, and generally seven rays. The species are quite numer- 

 ous, and found in all tropical seas; and one species, Jlolocentrus ascencionis or penta- 

 canthtts, occurs in the waters around Florida. It is an active fish, and its bright color 

 and quick, darting motions render it " one of the most conspicuous denizens of the 

 rock jDools." The common name, squirrel-fish, is supijosed to be due to sounds uttered 

 by the fish, which are imagined to resemble the bark of a squirrel ; and perhaps the 

 red color is another reason for the application of the name. 



Closely "related to the Holocentridse are families known as Beeycid^, Monocen- 

 TEiD^, and Stephanobektcid^. The representatives of these families are mostly 

 deep-water fishes which rarely come under the observation of the ordinary traveler or 

 collector. 



Next, we may briefly consider some of a long series of forms which are typified by 

 the common mackerel, and which have been kept together under the super-family 

 name SCOMBROIDEA. The representatives of this combination were first collected 

 together and segregated by Cuvier as a family designated Scomberoides, and were, 

 according to the great French naturalist, a multitude of fishes with small scales, a 

 smooth body, numerous caecal appendages often compacted together, and whose tail 

 and especially caudal fin are very powerful. The reasons given for the combination 

 are certainly not very satisfactory ; but that most (excluding several) of the types so 

 approximated are really closely related appeare to be clear, on comparison of the sev- 

 eral constituents ; and the proposition to intercalate them with other types, and scat- 

 ter them in widely distinct groups, seems to be a retrograde course. The unctious, 

 smooth body, imarmed scales, slender caudal peduncle, and powerful forked caudal 

 fin, are reinforced by features of development and structure. The preoperculum, for 

 instance, in very early youth, is armed with radiating spines, which are soon lost ; the 

 vertebrae and ribs have common characters, contrasting at least with those of the 

 forms with which they have been sometimes associated ; and develoisment of numer- 

 ous csBca, although perhaps of not much importance by itself, is a common feature, 

 whose generality (not constancy) is somewhat significant. Further, the caudal 

 peduncle, or base of the fin, is frequently strengthened by a longitudinal keel on the 

 side, and oblique ones above and below, either alone or together. Nevertheless a 

 good diagnosis of the group I'equires yet to be formulated. The group is still rather 

 the result of tact in appreciation than of scientific analysis. 



Many of the Scombroidea are of economical value ; and one of the most important 

 of food fishes has given name to the super-family. Many, if not most, of the species 

 are pelagic, or inhabitants of the high seas ; and among them we find those fishes 

 which pre-eminently embody the idea of the fish form, — fusiform, with a powerful, 

 forked tail-fin, and with a pointed head, all qualifying them in a superlative degree for 

 rapid horizontal progression through the water. 



Before reviewing the typical and specialized representatives of this great group, it 

 will be convenient to consider a form which has been almost always closely associated 

 with the CarangidsB, and which, in fact, does appear to be closely related, either as 

 nearer the primitive or as a derivative form. Reference is had to the notorious blue- 

 fish, the type and only known representative of a peculiar family, the Pomatomid^. 

 The body is oblong, fusiform in outline, but considerably compressed, covered with 

 scales arranged in oblique series, and with the lateral line running high up on the 



