520 Richard Owen, Esq., on the 



ginal separate condition throughout the vertebral column in the Plesiosaur, 

 and pass by such imperceptible gradations from one condition of physiolo- 

 gical subserviency to another, that their nature cannot be mistaken when the 

 entire series is viewed in a complete skeleton ; although, when viewed in 

 detached vertebrae of the neck or tail, they present the appearance and have 

 been generally described as hatchet-bones or transverse processes. 



True transverse processes are, however, always exogenous, or mere pro- 

 jections from the centrum or neurapophyses, and are of secondary importance. 

 They are of two kinds, superior and inferior; both are present in the cervical 

 vertebra3 in most classes ; the inferior transverse processes alone are deve- 

 loped in fishes. 



The oblique, or articuiating processes, are also exogenous, and may be 

 developed either from the neurapophyses or the base of the superior spines 

 of the vertebrae. 



As in other complicated bones resulting from an association of several 

 osseous pieces, certain elements of a vertebra may be modified in position 

 and proportions so as to perform the ordinary functions of others, which may 

 be atrophied or absent : thus in fishes, the inferior transverse processes are 

 gradually bent downwards, until, in the caudal region, their extremities meet 

 and perform the functions of the haemapophyses. 



The costal processes or ribs are considered by Geoffroy St. Hilaire* to un- 

 dergo in the Cetacea, a similar change of direction, and also a dislocation from 

 their usual attachments ; and to have their distal extremities bent downwards, 

 and anchylosed to a rudimental spine, so as to assume the form and perform 

 the offices of chevron-bones or hgemapophyses ; but as the horizontal pro- 

 cesses of the caudal vertebrae in the Cetacea (as exemplified in the skeleton 

 of a young Balcena Antarctica, fourteen feet in length, which I have lately 

 had the opportunity of examining,) are originally developed from distinct 

 centres, and in distinct cartilages, they appear to me to represent, with the 

 corresponding permanently separate vertebral elements in the Plesiosaurs, 

 the true costal appendages of the tail ; and the haemapophyses must, therefore, 

 be regarded as other and diff'erent elements of the vertebra. This view is 

 supported by the fact, that the long transverse processes supporting the ribs 

 in the thoracic region of the spine in the same young Whale, have no osseous 

 nuclei developed in them, but are continuous cartilages from the still unossi- 

 fied parts of the centrum. I may observe also, that the haemapophyses in the 

 young Cetaceans examined by me, exhibit what appears to be their permanent 

 condition in the Enaliosaurians, viz. a want of bony union at their distal extre- 

 * Mem. du Museum, ix. p. 118. 



