LOCOMOTION OF FISHES. 247 



middle and at the sides of the )3ody. Tlie protection against the 

 water-loo-o-ed timber and stones hurried alon<T their I'ecdino- 

 grounds, whicli the sturgeons derive from their scale-armour, 

 renders needless the ossification of the cartilaginous case of the 

 brain or other parts of the endoskeleton ; and the weight of the 

 armour requires that endoskeleton to be kept as light as may be 

 compatible with its elastic property and other functions. The 

 sturgeons are further adjusted to their place in the licjidd element, 

 and endowed with the power of changing their position and rising 

 to the surface, by a large air-bladder. 



These teleological interpretations of the dermal bony plates 

 may give some insight into the habits and conditions of existence 

 of those Ganoid and heavilj'-protccted Plaooid Fishes wMch so 

 predominated in the earlier periods of animal life in OTir planet, 

 which Ganoids and Placoids have hitherto been viewed almost 

 exclusively fiy the light of the analogy of an embrycjnic ' Age of 

 Fishes,' or explained as arrested stages in the ti'ansmutation of 

 Crustacea. I long ago demonstrated that both placoid plates and 

 ganoid scales, in the extinct {Lcjridotus^) as well as existing 

 (Lepidosteus) iishes, differed from the su})erficial shells of tlie 

 Invertebrata ^ in presenting the same organisation for growth and 

 repair, the same essential microscopic structure, as the ossified 

 parts of the endoskeleton which they serve to protect. 



The Cocciistcus, fig. 127, of the Old Red Sandstone, like the 

 Piiiidodus of the Ganges, had a half suit of such organised 

 armour; and, as Hugh JNIiller' suggests, the habits of the modern 

 sheat-fish may have l)een foreshown in priine\'al times by the 

 placoganold, burying the undefended jiart of its body in the mud, 

 and exposing ovllj its helm and cuirass, to arrest, as they passed, 

 the smaller animals on which it preyed. 



Nevertheless, the degree in which the exoskeleton jiredominates 

 <jver the endoskeleton as we penetrate into past time, descending 

 into the fossiliferous strata of the earth for evidence of ancient 

 life, is highly interesting and suggestive. 



At the present day only two lepidoganoid genera of fishes are 

 known — the Lepidostens of North America, and the Pohjpterus of 

 vlfrica — both restricted to fresh waters. Other existing fishes of 

 cognate organisation (Am/a, S'lidi,':, e. g. ) have soluble and flexible 

 scales. As we descend to the older tertiary beds the number of 

 Lepidoganoids increases, their geographical relations expand, and 

 their sphere of life embraces the salt waters of the ocean. At 

 the present day the jjlacoganoid and placoid, or plagiostomous, 



' V. p. 14. '' xxvu. p. 337. ^ cxcvi. p. 288. 



