FISHES. 283 



The Order CYCLOID. Have the body coveredwith circular 

 or elliptical scales of a simple homy or osseous structure 



FIG. 193. The Pike (Esox lucius}. 



without enamel, and with the posterior border nearly smooth. 

 The external surface of the scale is often marked in relief, with 

 the lines of growth (fig. 187). The scales of the lateral line 

 form tubes through which the mucus flows, that varnishes the 

 surface of the body of these fishes. The skeleton is osseous. 

 This order includes many of the malacopterygiens and acan- 

 tliopterygiens of Cuvier ; the Salmon, the Carp, and the Pike 

 (fig. 193), maybe regarded as its typical forms. The cycloid 

 order was created during the deposition of the flinty chalk of 

 the cretaceous period. It contains no extinct family, but a 

 great number of extinct genera which are most numerous in 

 the ancient strata. Thus nearly all the cycloid genera of the 

 chalk are extinct, and more than one half of those of Grlarus 

 and Monte-Bolca are so likewise. As we ascend through 

 the tertiary stages, the extinct genera diminish, and the 

 cycloid ichthyolites of these beds belong for the most part 

 to extinct species of existing genera. 



Order CTENOID. Have horny scales, composed of 

 plates : the posterior borders of which are denticulated ; the 

 plates are of unequal size, and are super-imposed on each ; 

 and as the inferior always exceeds the superior layer, the 



