VERTEBRAL COLUMN OF FISHES. 59 



to which are added in the abdominal region of most fishes two pleura • 

 popliyses {pi), or vertebral floating ribs. 



Ossification commences in the bases of the two neurapophyses and 

 the two parapophyses, and in the terminal concave plates of the cen- 

 trum ; the intermediate part of the centrum is sometimes completely 

 ossified, when it is filled by a coarse cancellous texture. More com- 

 monly a communicating aperture is left between the two tei'minal 

 concavities, (as indicated by the dotted line mjig. 16.) ; and, in many 

 cases, the plates by which calcification attains the periphery of the 

 body leave interspaces permanently occupied by cartilage, forming 

 cavities in the dried vertebra;, especially at their under part, or giving 

 a reticulate surface to the sides of the centrum. The expanded bases 

 of the neur- and par-ajiophyses usually soon become confluent with the 

 bony centrum : sometimes first expanding so as wholly to enclose it, 

 as, for example, in the Tunny, where the line of demarcation may 

 always be seen at the border of the articular concavity, though it be 

 quite obliterated at the centre, as a section through that part demon- 

 strates. 



In the Pike the neurapophyses seldom, in the Polypterus never, 

 coalesce with the centrum: the letter s shows the neurapophysial suture 

 vafig. 17. In the Salmonidce the parapophyses remain, for some time, 

 distinct from the body of the vertebra as well as from the ribs. In the 

 anterior vertebrae of the Carp the neurapophyses remain distinct, as 

 they do in the atlas of many other fishes, and a suture is observable 

 between the parapophyses and centrum in embryo Cyprinoids.* In each 

 vertebra the summits of the two neurapophyses usually become an- 

 chylosed together, and to their spine ; but in the Lepidosiren {Jig. 27.) 

 the spine retains its character as a distinct element, and is always at- 

 tached by ligament to the tops of the neurapophyses, as it is in the 

 Sturgeon {Jig. 12.). In the anterior abdominal vertebrae of the Tetro- 

 don, each of the neurapophyses, though they coalesce in the interspace 

 of the two spines to form the roof of the neural canal, sends up its 

 own broad truncated spine, and these are not, as might at first sight 

 be supposed, enormously developed oblique processes, for they gra- 

 dually approximate and blend together, to form the single normal 

 spine at the sixth abdominal vertebra : in the Barbel the neural 

 arches also support two spines, but one is placed beliind the other. 



The interspaces of the neural arches are occupied by a fibrous 

 aponeurosis — the remains of the primitive essential covering of the 

 neural axis : but in most fishes the arches are additionally con- 

 nected together by articular or oblique processes (zygapophyses), 

 which are developed from the base of each neurapophysis ; sometimes 



* First noticed by Von Baer. 



