By J. C. MANSBL-PLBYDELL, Esq., 

 F.G.S., F.L.S. 



ISHES arc the first vertebrate animals provided 

 with gills during the whole of life. Their form is 

 adapted for moving in water with the least 

 amount of resistance ; their vertebras, with few 

 exceptions, are hollow at both ends, which sub- 

 stitute the notochord of the less developed forms. 

 The lowest type is the Lance/et, whose vertebral column is 

 represented by a single notochord extending through the whole 

 length of the body, the extremities similar without any enlarge- 

 ment at the anterior end, as in the higher developed form. They 

 have only a membrane-cartilaginous skeleton without vertebra), 

 ribs, or jaws. The succeeding Order, including the Lampreys and 

 Hag-fish, have no hard structure except their small horny 

 teeth. Owing to the perishable structure of these fish they 

 would leave no evidence of their presence. The earliest true 

 fish-like vertebrate is Scaphaspis ludensis, from the lower Ludlow 

 of Leintwardine, Shropshire. The next Order, Plagiostomes, com- 

 prises the Sharks and the Rays. These have no continuous 

 covering of scales or ganoid-plates, but of more or less detached 

 tubercles or spines, scattered here and there on the skin. They 

 have neither gill-covers nor operculum, but branchial slits, which 



