142 LECTURE \a. 



{fig. 11.); Ijut the true liomologues of the sternum are first seen in 

 the endo-skeleton of the Batracliia. In the Trunk-fishes {Ostracion\ 

 and Pipe-fishes i^Syngnatlms) the dermal scale bones form a con- 

 tinuous coat of mail, like a tessellated quincuncial pavement, over the 

 entire body. In the Lepidosteus the scales defend the body in close- 

 set oblique rows, are thick, completely ossified, and with an exterior 

 hard, shining, enamel-like layer, having the microscopic structure of 

 the hard dentine of Shark's teeth ; the subjacent osseous part exhibits 

 the radiated corpuscles. I described the organic structure of these so- 

 called ' ganoid' scale bones in 1840, in both recent and extinct 

 fishes, showing that it militated against the theory of development 

 by successive deposition of layers being applied, at least, to ganoid 

 scales. * A like organisation jirevails in the tri-radiate dermal bones 

 which support the strong spines of the Diodon ; and in the usually 

 unenamelled, less regularly formed and arranged, dermal 'placoid' 

 ossicles of Sharks and Rays. The thinner subtransparent scales of 

 ordinary Osseous Fishes are either sub-circular and with entire 

 margins as in the Carp, when they are called ' cycloid,' or have the 

 outer and hinder margin dentated or spined, as in the Perch, when 

 they are called ' ctenoid.' We have seen that the primary classi- 

 fication of fishes in the system of M. Agassiz, is based on these 

 various modifications of the dermal skeleton. 



One of the interesting generalisations which has risen out of the 

 vast series of researches on Fossil Fishes to which this eminent 

 Naturalist has devoted himself, is the discovery of the progressive 

 predominance of the exo-skeleton over the endo-skeleton as we 

 descend into the strata of the earth, or, in other words, penetrate into 

 past time in quest of the species that have been successively blotted 

 out in the revolutions of the globe. At the present day the Placoids 

 or Plagiostomous cartilaginous fishes form a small minority of the 

 class ; and amongst the existing majority of fishes called, from the 

 advanced development of their internal skeleton, ' Osseous,' only two 

 genera exhibit that kind of scale called ' ganoid : ' one of these, the 

 Lepidosteus, is peculiar to North America ; the other, the Polypterus, 

 to Africa : both are fresh-water fishes. As we descend to the older 

 tertiary deposits the number of Ganoid Fishes increases, their geo- 

 graphical relations expand, and their sphere of life was extended to 

 the salt waters of the ocean. 



Thus Ichthyolites with a dense imbricated armour of polished bony 

 scales occur in the marine deposits of the eocene age in our own 



* Odontography, part i. p. 15. 



