5 10 



GANOIDS, 



(Dapediidce) is represented by Dapedius, Lepidotus,- and several other allied 

 genera, in which the body is more or less deeply fusiform, the suspensory apparatus 

 of the lower jaw either vertical or inclined forwards, the cleft of the mouth narrow, 

 the teeth cylindrical or in the form of button-like knobs, the vertebrae not more 





HSH^^ feSSssi^^ 



,. \s?'' 



QQO g 



THE GIANT SCALE-TOOTH, WITH A DETACHED SCALE AND TEETH 



(much reduced). 



than rings, and the 

 dorsal fin not ex- 

 tending more than 

 half the length of 

 the bod}^. In this 

 family the teeth 

 have vertical suc- 

 cessors ; and while 

 some of the earlier 

 genera date from 

 the Trias, the scale- 



tooths (Lepidotus), of which an example is figured in the illustration, survived till 

 the Chalk. Some of the species of this genus attained very large dimensions ; and 

 their remains are beautifully preserved in the Lithographic Limestone of Bavaria. 

 In all these the scales are of the typical quadrangular ganoid type. 



The Sturgeon-Tribe, — Suborder Chondrostei. 



This important suborder brings us to the last group of the fan-finned fishes 

 (Actinopterygii), which forms a division by itself differing in several important 

 particulars from the one including the whole of the foregoing suborders ; the more 

 important characters of the first division having been given on p. 334. Whereas 

 in that division the number of dermal rays in the dorsal and anal fins is equal to 

 the supporting elements in the true internal skeleton, in the present division the 

 dermal rays are more numerous than their supports. Then, again, whereas in the 

 former division the pelvic fins have their superior row of supporting ossicles, or 

 baseosts, rudimental or wanting, in the present group these are well developed. 

 The living representatives of the sturgeon tribe agree with the bow-fish and its 

 allies in the want of any interlacing of the fibres of the optic nerves at their crossing, 

 and likewise in the presence of a spiral valve to the intestine. In both the living 

 and extinct types the tail is of either the diphycercal or heterocercal type. As a 

 suborder, the sturgeon tribe may be characterised by the more or less completely 

 persistent notochord, by the inferior and superior supporting ossicles (axonosts and 

 baseosts) of the dorsal and anal fins forming a simple and regular series, and also 

 by the presence of a pair of infraclavicular plates in the pectoral girdle. In all 

 the known forms there is a single dorsal and anal fin, both of which are well 

 separated from the caudal ; while in the existing members the air-bladder is fur- 

 nished with a duct. Although represented at the present solely by the sturgeons 

 and their allies, the group was x<'ry abundant during the Secondary epoch : and 

 whereas the sturgeons, together with certain extinct families, form what may be 

 termed a degenerate specialised series characterised by the absence of ganoid scales 

 in a second and normal series the body was covered with such scales. 



