252 



REPORT — 1846. 



Fig. 16. 



their supporting skeleton in the primitive histological 

 fibrous state, the corresponding parts are ossified in fishes: 

 rarely, however, are such parts in answerable number to 

 the vertebrae ; and the true spines of these vertebrae, 

 when the median fins and their bony spines are removed, 

 in fishes, show as little indication of the place or existence 

 of such fins, as do the vertebrae in the porpoise of the 

 existence of its dermal fin. In proportion as ossification 

 has extended into the dermal system of fishes it has been 

 arrested in the vertebrae, which in the trunk and tail of 

 fishes present their least complex condition. Two of the 

 autogenous elements, the ' haemapophyses,' are absent, and 

 are commonly represented, in the tail, by the modified 

 ' parapophyses.' The seeming complexity of a fish's ver- 

 tebra arises from the intercalation of bones appertaining 

 to the system of the dermo-skeleton : it would have been an 

 unusual exception to the general course of development if 

 the lowest of the vertebrate classes should have presented 

 the vertebral skeleton in its highest state of complication ; 

 and Geoffroy St. Hilaire was unfortunate in taking a fish's 

 vertebra with its extrinsic evertebrate complications, as the 

 perfect type of that primary segment of the myelencepha- 

 lous skeleton (fig. 16). He was still more unlucky in having 

 for the subject of his figure* a specimen from which two 

 of the pieces had been accidentally lost, as Cuvier after- 

 wards pointed out ; yet Geoffroy 's mutilated caudal ver- 

 tebra of the plaice continues to be copied in some 

 compilations of comparative anatomy, as the type of a 

 vertebra ! To obtain the dermal spines (pro-epial and pro- 

 cataal) of the vertically extended caudal vertebrae of fishes, 

 Geoffroy had recourse to a hypothetical division length- 

 wise of the interneural and interhaemal spines (which are 

 represented as being single in his figure), and to as gra- 

 tuitous a displacement of one of the halves from the side 

 to the summit of the other f. Now the interneural and 

 interheemal spines are actually double in relation to the 

 neural and haemal spines ; yet they coexist with a dermo- 

 neural and dermohaemal ray, which therefore needs no 

 imaginary change of place of either of its supporting 

 spines to account for its existence. I subjoin in fig. 

 16 an entire vertebra answering to the mutilated one 

 figured by Geoffroy ; and for the better understanding of 

 the difference between his determinations of the vertebral 

 elements and those given in the present Report, the names 

 respectively indicating those different determinations are 

 added to the figure. In the description of the plate in 

 the ' Memoires du Museum,' Geoffroy explains that the 

 ' pro-epial' is the left half or 'epial gauche] and the en-epial 

 the right half or ' epial droit' : that the en-cataal is the right 

 half or ' cataal droit,' and the pro-cataal the left half or ' cataal gauche,' of his 

 imaginably divided epivertebral and catavertebral elements (I. c. p. 115). 



* Memoires du Museum, t. ix. (1822), pi. 5, fig. 1. 



f " L'une de ces pieces monte sur l'autre " — " l'une se maintient en dedans, quand 

 1'autre s'elance en dehors," ib. p. 97. 



Endo- and exo-ske- 

 letal elements of a 

 caudal vertebra of 

 a Plaice (Pletiro- 



nectes). 



