28 CLASSIFICATION OF FISHES. 



are so very short as just to appear above the scales^ they 

 have no membrane ; but if longer, a sUght one connects 

 each of them to the back, but not to each other : in 

 general the point is directed backwards ; but in some 

 few genera, allied to the mackerel, some of these prickles 

 are directed forwards, and others terminate in two or 

 three spines, or are bifid or trifid. The most remark- 

 able instance of these dorsal spined fishes is the genus 

 Acanthonotus, where there is a row of ten of them, de- 

 tached, placed both before the dorsal and the anal fins : 

 more familiar examples are seen in pur sticklebacks 

 (^Gasterosteus Linn.), of which the G. spinochia Linn, 

 has no less than fifteen before the dorsal. The spines in 

 the first dorsal fins of the acanthopterygeous fishes 

 are almost always graduated: the first being short, 

 while the second is intermediate between that and the 

 third ; which latter (or the fourth) is usually the 

 longest : in particular groups, however, there is always 

 some prevalent modification of this fin, which we shall 

 now notice. 



(31.) The shape or form of the dorsals is consider- 

 ably varied : where there are two or three, those which are 

 in front are almost always triangular, while the hinder 

 one is of more equal breadth throughout. In the common 

 cod {fig. 2. a), the first is acutely triangular, the two 

 next less so ; but in Blepsias, its representative among 

 the Canthileptes, the posterior of the three connected 

 fins is broadest in the middle {fig. 2. b). In Trachinus 

 and its numerous representatives, the first dorsal is short 

 and triangular, while the second is long and narrow (c). 

 In the mackerel family, however, where all the fins are 

 subfalcated, both the dorsals are consequently of the 

 same form ; but this comparatively is a very unusual 

 structure, although it affords an absolute character to 

 the Scomberidce (d). In the sharks, the mullets, and a 

 few others, where the two dorsals are wide apart, both 

 of them are triangular. Nearly all the typical Gymnetes 

 have the dorsal fin highly developed ; it is here also 

 sometimes particularly broad, with the anterior rays 



