248 ANATOMY OF VERTEBRATES. 



fishes, form a small minority of the class. In the chalk forma- 

 tions the number of the species of Placoids and Ganoids rapidly 

 increases, and soon preponderates ; in all the older fossiliferous 

 strata they exclusively represent the class of Fishes. The pre- 

 dominance of osseous matter deposited in the tegumentary system 

 in these ancient extinct Fishes is not unfrequently accompanied 

 by indications of a semi-cartilaginous state of the endoskeleton, 

 like that in the Lepidosiren of the present day ; the total absence 

 of any trace of vertebral centres, and the vacant tract, where they 

 should have been, between the bases of the neur- and hgem- 

 apophyses which have been little disturbed, as in fig. 127, show 

 plainly enough that the primitive gelatinous notochord has been 

 persistent. 1 In not one of the extinct Fishes of the Devonian 

 and Silurian systems has a vertebral centrum been discovered ; 

 but the enamelled dermal osseous scales and plates are richly 

 developed, and most remarkable for their beautiful and varied 

 external sculpturing, and often for their great size. 



In the mesozoic strata ganoid species exhibit a progressive 

 expanse and downward growth of the neurapophyses, converting 

 the notochordal capsule into distinct bony segments ; the terminal 

 cups of bone are subsequently added, and the centrum is completed. 



At the present day the Lepidosiren repeats the notochordal con- 

 dition of the endoskeleton, but without the compensating ganoid or 

 placoid developements of the skin; and the Sheat-fishes {Siluridce} 

 combine the large tuberculated osseous dermal plates with a well 

 ossified internal skeleton. The existing sturgeons alone manifest 

 contrasted conditions of the endo- and exo-skeletons, like those in 

 the ancient placoganoids ; but what is now a rare and exceptional 

 instance of analogy to the testaceous and crustaceous Inverte- 

 brates was the rule in the first-born fishes of our globe. 

 Those fishes, which from the determination of the ossifying 

 energies to the endoskeleton have been termed Teleostei, consti- 

 tute the bulk of the tertiary and existing species of this class. 

 But gradations of endoskeletal ossification are still indicated. 

 A great proportion of the primitive cartilage is retained in the 

 skull of many of the Teleostei, the Salmon and Pike, for example ; 

 and the greater proportion of the animal to the earthy matter in 

 all the bones, their coarse texture, the radiating fibres of the flat 

 cranial bones, and the general absence of dentated sutures, are 

 characters in all Osseous Fishes, which remind the Anthropotomist 



of transitional ones in the human foetus ; but the liijht of teleo- 



7 ~ 



1 See also the beautiful illustration of this fact in the Microdus radiatus, No. 626, 

 of the Hunterian Series of Fossil Fishes in the Museum of the London College of 

 Surgeons; cxcm. p. 155. 



