LEPIDOGANOIDEI. 169 



The heterocercal Ganoids are almost completely super- 

 seded, in the oolitic series, by homocercal genera, which now, 

 for the first time, appear on the stage of life ; but the ossifica- 

 tion of the endo-skeleton is still incomplete. In the cretaceous 

 series the Teleostian, or well-ossified, bony fishes, are nume- 

 rous; and here also first are seen fishes with the flexible 

 " cycloid" or " ctenoid" scales, and of genera which continue 

 to be represented by living species. 



In the tertiary division of geological time the ganoid order 

 rapidly diminishes, and its place is taken by fishes with better 

 ossified internal skeletons, and with thinner, more flexible, and 

 usually soluble scales. The gills are supported on bony arches, 

 and are protected by branchiostegal rays, and by an operculum 

 or gill-cover. The aortic bulb is provided with but two valves ; 

 and the optic nerves decussate. For this group, including the 

 majority of existing fishes, and of those which made their 

 appearance during the tertiary period, Muller proposed the 

 name " Teleostei," which almost corresponds with the " osseous 

 fishes" of Cuvier. The difference of shape of scale seems not 

 to have been more significative of ordinal distinction in the 

 fishes with flexible or horny, than in those with hard 

 enamelled scales ; the toothed border, or " ctenoid" type, of 

 the soft scale may serve, however, like the even border or 

 "cycloid" type, to characterize minor groups of the better 

 defined orders of soft-scaled fishes. 



Order IV.— ACANTHOPTERL 



Char. — Endo-skeleton ossified; fins with one or more of the 

 first rays unjointed or inflexible spines ; ventrals in most 

 beneath or in advance of the pectorals ; swim-bladder 

 without air-duct. 



