430 MANIPULATION. 



remain of the second class, almost all being extinct ; the Lepi- 

 dosteus, or Bony-pike, of North America, the Sturgeon, and 

 Paddle-fish, are the most familiar examples. A knowledge 

 of the form and structure of scales, like that of teeth, has, by 

 the labours of M. Agassiz, been shown to afford an unerring 

 indication of the particular class to which any fish may belong; 

 in fossU fish, the application of this principle has been attended 

 ,/ith extraordinary results. By Agassiz, the scales have been 

 divided into four orders, named Placoid, Ganoid, Ctenoid, and 

 Cycloid; in the first two, the scales are more or less coated 

 with enamel, whilst in the others they are of a homy nature. 

 To the Placoid order belong the cartilaginous fishes, whose 

 skins are either entirely or partially covered with small prickly 

 or flattened spines, as in the skates, dog-fish, and sharks. Of 

 the Ganoid order, once the most numerous, only a few living 

 representatives, such as the Lepidosteus, Polypterus, and 

 Sturgeon remain; the others are found in the fossil state 

 alone; their scales present a true bony structure. The 

 Ctenoid scales are notched like the teeth of a comb on their 

 posterior or attached borders, the perch and basse are excellent 

 examples; whilst to the Cycloid belong those fish whose scales 

 are more or less laminated and circular — the majority of our 

 edible fish, such as the carp, roach, salmon, herring, &c., 

 afford familiar illustrations of this order. The method of 

 mounting scales of various kinds for microscopic examination, 

 is generally in the dry way, either on discs or between glasses; 

 the former method is the best for those to be viewed as 

 opaque objects, the latter for transparent ones. Their struc- 

 ture, however, is best seen in fluid, when many of them will 

 form splendid objects for polarized light; for this purpose they 

 may even be mounted in balsam. Fragments of fossil scales 

 of fish are best prepared in the latter way; these may 

 generally be obtained from nodules of flint, found in particular 

 localities. According to the author of the work entitled 

 Microscopic Objects, " those from the gravel drifts at Gilling- 

 ham, in Kent, and the flint nodules in the chalk between 

 Gravesend and Rochester," seldom fail, when broken into flat 

 pieces, to yield an abundant supply A few of the most 



