ICHTHYOLOGY. 



217 



In Ganoid fishes the plates or scales are covered by a the head to the caudal fin, or even through the latter, called Introduc- 



•^ tion. 



Dermal 



lA^^''' ^^^- W'^ R*^ '■ fv?' >^^S^i^fe, Skeleton. 



Fig. 37. 



Ctenoid scales from the lateral line at the base of the caudal fin of Caronx pa- 



rapistes, a Scomberoid from North Australia. 



the lateral line. It is nearer the back in some fishes than in 



Fig. 33. 

 Scales of Macrourus coElorhynchua. 



thick coat of enamel, and are sometimes of considerable di- 

 mensions ; and in many of ~. 

 the extinct species which 

 compose the bulk of this 

 order the enamelled plate is 

 rhomboidal, with a hook at 

 its anterior angle, which 

 assists in retaining it in its 

 place. The recent species 

 included by Agassiz in this 

 order do not form a 

 natural group. 



Fishes of the Placoid 

 order have the skin covered 

 witli irregular plates of 

 hard bone, varying greatly 

 in shape and size. In the 

 Rays and Monk fish they 

 form large, rough, or spinous 

 tubercles ; and in the Dog- 

 fish, fine-grained shagreen. 

 As formed by Agassiz, the 

 order corresponds with the 

 Cartilaginei or Cliondropterygii of authors, and includes 

 species with smooth skins. For the arrangement of fossil 



Fig. 34. 

 Scale of Macrourus trachyrhynchu.t. 



Fig. 35. 

 Placoid scales of Aleuterestrossulus, one of the BalistidaBt from Australia. 



fish these divisions were well devised, but, as may be ex- 

 pected, a system based on the variable characters of a 



Fig. 36. 

 Scales from different parts of the body of Alcuteres variabilis. 



portion of the dermal skeleton fails in defining natural 

 groups. Used in conjunction, however, with the other 

 parts of the structure, Agassiz's discoveries are of great im- 

 portance for the purpose of classification. 



Along the sideofan osseous fish, there is a line running from 

 VOL. xu. 



Fig. 38. 



Cycloid scale from the lateral line of Labrus laiiclavius, a.n Australian 



Cyclo-Labroid. 



Others ; very generally it runs at mid-height, sometimes it 



ceases a long way before 



the caudal fin ; and in 



Cliirus there are several 



lateral lines, the upper 



one coasting the back, 



and the lower one the 



edge of the belly. This 



line is nniciferous, being 



connected with a series jig g,,. 



of glands. In thej4«(/M2/-Cvcloid scale from the lateral line of Odax 

 IcB, the mucous ossicles "««<""'. a. ijonus closely allied to Soa,™. 



lying like the scales under the cuticle are scarcely visible, 

 and in many scaleless vermiform apodals the line is still less 

 conspicuous ; but in these 

 slimy fishes the whole skin is 



densely studded with glands ,^^^5 \ 



and their ducts. The scales ^S~i? 

 that cover this line, or lie in tr 

 immediate contact with it, 

 above and below, are con- 

 structed with a tubular arch 

 along their middle, or a 

 simple perforation in the disk, 

 or with a mere notch on the 



edge for the exit of the mu- 



coduct (figs. 2.J, 29, 32, 38, Fig.40. 



39 40 41 43 and 44). The a Cycloid scale from the lateral line of 



', ' ' ' . , Lutodeira salmon€a,one oa\\e GonO' 



scales are sometimes larger rhynckidx from Torres straits and 



than the ordinary ones in the Polynia- 



same fisli, occasionally they are smaller, and sometimes they 



:i-^^i 

 'p^ 

 ^ ^*» 



Fig. 41. 



Fig. 42. 



Fig. 41. A cycloid scale from the lateral line of Saurus undosquamiSt one of 

 tiie North Aiislralian ScopcLina:. 

 ... 42. ctenoid scale oi I^latycephalus cirrhonasuSt ODO of the Sclerogenidw of 

 Botany Bay. 



are the only ones, the rest of the body being naked. Very 



2e 



