PLACOGANOIDEL 147 



above plates in the side view (fig. 61), with the addition 

 of (12) the dorsomedian and (14) the post-dorsomedian. 



The blank space between the neural (n) and haemal (h) 

 spines of the fossil endo-skeleton indicates the position of the 

 soft "notochord" (c), which has been dissolved away. The 

 cylindrical gelatinous body, so called (in Latin chorda dorsalis) 

 pre-exists to the formation of the bony bodies of the vertebrae 

 in all vertebrate animals ; and the development of those bodies 

 seems never to have gone beyond this embryonal phase in any 

 palaeozoic fish ; such fishes are accordingly termed " noto- 

 chordal," as retaining the notochord. 



There are but two genera of existing fishes wdiich mani- 

 fest, when full grown, such a structure, associated with 

 ossified peripheral elements of the vertebrae — viz., the Proto- 

 ptevus of certain rivers of Africa,* and the Lepidosiren of 

 certain rivers of South America. Those fishes, if fossilized, 

 would present the appearance of the vertebral column shewn 

 in fig. 61 : and the like persistence in all palaeozoic and most 

 mezozoic (figs. 72 and 74) fishes of an embryonic vertebral 

 character, transitory in nearly all existing fishes, significantly 

 testifies to a principle of " progression." 



The external ganoid surface of the buckler plates of 

 Coccosteus is ornamented with small hemispherical tubercles ; 

 whence the generic name, signifying "berry-bone." The 

 similarity of this ornamentation to that of the plates of the 

 buckler in some tortoises, led to the belief, when the coccos- 

 teal plates were first found, of their being evidence of the 

 chelonian genus Trionyx in Devonian beds. Passing notions 

 also got into print of the crustaceous affinities of Coccosteus ; 

 whence the trivial name of the type-species decipiens, or the 

 " deceiving" Coccosteus. 



Strange as seem the forms and structure of the placo- 



* See Linnsean Transactions, vol. xviii. ; and Proceedings of the Linnsean 

 Society, April 2, 1839. 



