422 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. V. 



corneous plates covered with a dense coat of enamel, which 

 form a dermal integument of great strength and solidity. 

 In many of the most ancient types, the body is literally enve- 

 loped in an osseous case ; the bones of the cranium coalescing, 

 and the scales of the thoracic, dorsal, and abdominal regions, 

 blending as it were into a cuirass; hence the fishes of this 

 order, which are among the most ancient known types of ver- 

 tebrata, appearing in the Devonian or Old Red formation, afford 

 the only absolute knowledge we possess of the earliest forms and 

 structures of Ichthyic organization ; for of the Placoids, which 

 appear in a still earlier geological epoch, namely, the Silurian, 

 owing to the cartilaginous and perishable nature of their 

 skeletons, a few rays or spines, teeth, scutcheons, and shagreen 

 skins, are the only vestiges that remain in a fossil state. 

 Thus the minutely dentated fin-ray of the ffomocanthus, of 

 the Devonian formation, is the only fossil relic of that placoid; 

 while in its contemporary ganoid fish, the Osteolepis, indica- 

 tions of the structure of the organs of smell, hearing, and vision, 

 are manifest. 



The Ganoid order comprises twelve families, examples of 

 which are arranged in Wall-cases A and B, and are comprised 

 in the subdivisions marked 1 to 1 6 in the compartments of 

 the glass-cases; as enumerated in the list, ante, p. 416. 



CEPHALASPIDIANS. Wall-case A. [1.] The Devonian 

 Formation, (see Geological Table, ante, p. 5,) in which, but 

 twenty-five years ago, a few single scales, discovered in Forfar- 

 shire by Dr. Fleming, were the only known traces of any 

 vertebrated animals, has yielded upwards of sixty species 

 belonging to nearly thirty genera, from British localities 

 alone. 



Of these the most characteristic are the CepJialaspis, Pte- 

 richthys, and Coccosteus, which form a group of extinct genera 

 that has no representative either in the Silurian system below, 

 or in the Carboniferous above ; nor, except by distant and faint 

 analogies with existing fishes, can these anomalous organisms 

 be brought within the pale of zoological arrangement. These 

 ichthyolites agree in one general character, that of being 

 covered by relatively enormous osseous or horny plates, and 

 scutcheons. 1 No vertebrae have been found, and it is supposed 



1 See " Medals of Creation," p. 645 ; "Wonders of Geology," p. 760. 



