ON THE VERTEBRATE SKELETON. 259 



tunny, and salmon yield this striking refutation of the idea of the formation 

 of those arches in all fishes, by displaced, curtailed and approximated ribs. In 

 some fishes, however (e. g. the cod), reduced pleurapophyses coalesce with the 

 parapophyses to form the haemal arches of the caudal vertebrae. The meno- 

 pome, amongst the lowest or perennibranchiate reptiles, yields a clear disproof 

 of the formation of the haemal arch in the tail by the pleurapophyses (the 

 parts, viz. called by Geoffroy ' paraux,' and by Dr. Grant ' catavertebral ele- 

 ments ' in the abdomen of fishes)*. The vertebral ribs or pleurapophyses in 

 the menopome (fig. 27? pV) are short and simple and suspended to the extre- 

 mities of the diapophyses (d) at the beginning of the tail, where they coexist 

 with haemal arches (h, K) : these must be formed, therefore, by different ele- 

 ments, which, since no trace of parapophyses exists in any part of the spine, 

 I conclude to be the ' haemapophyses.' The young crocodile and the adult 

 enaliosaurs give the same evidence of the nature of the haemal arches in the 

 tail, with which the corresponding arches or chevron-bones, in cetacea and 

 many other mammalia, are homologous. 



Thus the contracted haemal arch in the caudal region of the body may be 

 formed by different elements of the typical vertebra : e. g. by the parapophyses 

 (fishes generally) ; by the pleurapophyses (lepidosiren) ; by both parapophy- 

 ses and pleurapophyses (Sudis,Lepidosteus), and by haemapophyses, shortened 

 and directly articulated with the centrums (reptiles and mammals) f. The 

 caudal vertebrae of some flat-fishes (Pleuronectidcs, fig. 16), and the mu- 

 raenee, would seem to disprove the parapophysial homology of the haemal arches 

 in such fishes, since transverse processes from the sides of the body coexist 

 with them, as they do in the cetacea. But, if we trace the vertebral modifi- 

 cations throughout the entire column in any of these fishes, we shall find that 

 the haemal arches are actually parts of the transverse processes ; not independ- 

 ent elements, as in the cetacea ; but due to a progressive bifurcation : this, in 

 Murcena Helena, for example, begins at the end of the transverse processes 

 of about the twenty-fifth vertebra, the forks diverging as the fissure deepens, 

 until, at about the seventy-third, the lower fork descends at a right angle to 

 the upper one (which remains to represent the transverse process), and, 

 meeting its fellow, forms the haemal arch, and supports the antero-posteriorly 

 expanded haemal spine. In the plaice a small process is given off from the 

 expanded base of the descending parapophysis of the first caudal vertebra, 

 which increases in length in the second, rises upon the side of the body in 

 the third, becomes distinct from the parapophysis in the fourth, and gradually 

 diminishes to the ninth or tenth caudal vertebra, when it disappears. These 

 spurious transverse processes never support ribs. 



The neurapophyses are often directly perforated by the nerves in fishes, 

 but are sometimes notched by them, or the nerves issue at their interspaces. 



The neurapophyses, which do not advance beyond the cartilaginous stage in 

 the sturgeon, consist in that fish of two distinct pieces of cartilage; and the an- 

 terior pleurapophyses also consist of two or more cartilages, set end on end : and 

 this interesting compound condition is repeated in cases where the pleurapo- 

 physial element is ossified and required to perform unusual functions in the 

 bony state in other fishes. Amongst the more special or exceptional modifi- 

 cations of the vertebrae of the trunk of fishes, which indicate the extent to 

 which their normal segmental character may be marked, I would cite those 

 of the anterior vertebrae in the pipe-fishes, in the loaches, and in certain 

 siluroids. 



* Outlines of Comparative Anatomy, p. 58, fig. 28, B, g. 



f By a misconception of the sense in which I use the term ' haemapophyses,' M. Agassiz 

 has applied it to the laminae of the inferior or haemal arches in fishes. " Recherches sur les 

 Poiss. Foss." torn. i. p. 95. 



