DERMAL BONES OF FISHES. 143 



island. In the chalk formations the members of both Ganoids and 

 Placoids multiply rapidly, and in all the older fossiliferous strata 

 they exclusively represent the class of fishes. The predominance of 

 osseous matter deposited in tlie tegumentary system in tliese ancient 

 extinct fishes is not unfrequently accompanied by indications of a 

 semi-cartilaginous state of the endo- skeleton, like that in the Lepido- 

 siren of the present day ; the total absence of any trace of vertebral 

 centres in this fossilised skeleton of the Microdon radiatus (No. 70. 

 Fossil Fishes, Mus. Coll. Chirurg.), and the vacant tract, where they 

 should have been, between the bases of the neur- and haema-pophyses 

 which have been little disturbed ; together with the remains of tlie 

 ganoid scale-armour which has kept all the fossilisable parts of the 

 extinct fish together, show plainly enough that the primitive gela- 

 tinous chorda dorsalis has been persistent. In not one of the nu- 

 merous 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 Coccosteus they form a broad helmet upon tlie head, 

 and a back -plate and breast-plate for the fore part of the trunk, and 

 have been mistaken for the scutes of a Tryonyx or Mud-tortoise ; 

 whilst only the periplieral arches and spines of the vertebrae of this 

 fish were ossified, and a great proportion of the cranial vertebras was 

 cartilaginous. In the still better defended Pterichthys and Pam- 

 phractus*, which have been mistaken for extinct Crustacea, all the in- 

 ternal skeleton was soft and perishable, and the earthy salts were ex- 

 clusively developed in that peripheral skeleton, which forms the sole 

 calcified defence of the invertebrate classes of animals. It is a striking 

 and suggestive fact this prevalence of a low and rudimental state of the 

 endo-skeleton, with an excessive development of the exo-skeleton, in 

 the fishes of the old Silurian and Devonian strata — the earliest 

 periods at which Geology teaches that fishes were introduced into this 

 planet. At the present day the Lepidosiren repeats the low con- 

 dition of the endo-skeleton, but without the compensating ganoid or 

 placoid developments of the skin ; and the Siluroids 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 Cephalaspids ; 

 but what is now a rare and exceptional instance of analogy to the 



* See RI. Agassiz' admirable, ijliilosophical, and splendidly illustrated mono- 

 graph, " Sur les Poissons Fossilcs du Systcme Devonien," 4to. tab. 21 — ;51. 



