62 PICTURESQUE SKETCHES 



extent of the difference that existed between those of 

 the earlier seas and the present time. 



The result of this, and the conclusions arrived at 

 by a careful and minute study of the natural history 

 and anatomy of fishes, has been lately laid before the 

 public by M. Agassiz, a naturalist whose great acute- 

 ness of observation and knowledge of the structure 

 of fishes, have long been well known to the scientific 

 world, and were appreciated by Cuvier, who left in 

 his hands the papers he had himself accumulated on 

 the subject of Ichthyology. 



According to M. Agassiz, fishes may be collected 

 into four natural orders, two of which have been alrea- 

 dy mentioned as including most of the recent fishes, 

 while each of the other two groups, so rarely met 

 with at present, contains species representatives of an- 

 other order, equally important and well marked, and 

 in former times represented almost to the exclusion 

 of any species of the two orders now so abundant. 

 The first of the two groups, that of which the stur- 

 geon and the bony pike are characteristic, is called 

 GANOID ; while the other, containing the sharks and 

 rays, is known as the PLACOID order *. Of these, the 

 Placoid seems to have been the first introduced, but 

 the Ganoid was that which attained its greatest de- 

 velopment in the ancient seas.-)- 



* Ganoid, from the Greek yavoQ (ganos\ splendour ; the scales of these 

 fishes being generally coated with polished enamel, and often exhibiting 

 a very brilliant lustre. Placoid, from 7r\a (plaai), a plate or slab; be- 

 cause the skin of the animals of this order is irregularly covered with 

 plates, studded often with enamel. (See figures 20, 21.) 



f The remaining two groups are called respectively Ctenoid (itrevof, 

 ctenos, a comb) and Cycloid ( KVK\O, cyclos, a circle), from the shape and 

 structure of the scale. (See figures 18, 19.) 



