ON THE VERTEBRATE SKELETON. 255 



laterally projecting parapophyses in the first twelve or twenty abdominal ver- 

 tebrae : in the anterior ones these ' vertebral ribs ' are composed of two or 

 three distinct cartilages* : the posterior pleurapophyses are short and simple. 

 The parapophyses gradually bend down to form haemal arches in the tail, at 

 the end of which we find haemal cartilaginous spines corresponding to the 

 neural spines above. The tapering anterior end of the notochord is con- 

 tinued forwards into the basal elements of the cranial vertebrae. Vegetative 

 repetition of perivertebral parts not only manifests itself in the composite 

 neurapophyses and pleurapophyses, but in a small accessory (interneural) car- 

 tilage, at the fore and back part of the base of the neurapophysis; and by a 

 similar (interhaemal) one at the fore and back part of most of the parapo- 

 physes f. 



Amongst the sharks (Squalidce) a beautiful progression in the further 

 development of a vertebra has been traced out, chiefly by J. Miiller £. In 

 Heptanchus (Squalus cinereus) the vertebral centres are feebly and vege- 

 tatively marked out by numerous slender rings of hard cartilage in the noto- 

 chordal capsule, the number of vertebrae being more definitively indicated by 

 the neurapophyses and parapophyses ; but these remain cartilaginous. In 

 the piked dog-fish (Aeanthias) and the spotted dog-fish {Scylliiim) the ver- 

 tebral centres coincide in number with the neural arches, and are defined by 

 a thin layer of bone, which forms the conical articular cavity at each end: 

 the whole exterior of the centrum is covered by soft cartilage, except at the 

 concave ends ; the two thin funnel-shaped plates of osseous matter coalesce 

 at their perforated apices, and form a basis of the vertebral body like an 

 hour-glass ; the series of these centrums protecting a continuous moniliform 

 remnant of the gelatinous notochord. In the great basking-shark (Selache) 

 the vertebral bodies are chiefly established by the terminal bony cones, the 

 thick margins of which give attachment to the elastic capsules containing 

 the gelatinous fluid, which now tensely fills the intervertebral biconical spaces. 

 Four sub-compressed conical cavities extend, two from the bases of the 

 neurapophyses, and two from those of the parapophyses, towards the centre 

 of the vertebral body, contracting as they penetrate it. These cavities always 

 remain filled by a clear cartilage : the central two-thirds of the rest of the 

 vertebral body contain concentric, progressively decreasing, and minutely 

 perforated rings or cylinders of bone, interrupted by the four depressions : 

 the peripheral third of the vertebral body contains longitudinal bony laminae, 

 which radiate, perpendicularly to the plane of the outermost cylinder, to the 

 circumference ; these outer laminae lie, therefore, parallel with the axis of the 

 vertebra, and the intervening fissures, like those between the concentric cylin- 

 ders within, are filled by clear cartilage, which shrinks, and leaves them open 

 in the dry vertebra §. 



In Cestracion, the intermediate part of the centrum between the terminal 

 cones is strengthened by longitudinal radiating plates only ; in Squatina by 

 concentric cylinders only. In the tope (Galeus) all the space between the 

 terminal bony cones is ossified, except the four conical cavities, the bases 

 of which are closed by the neur- and par-apophyses ; so that the whole 

 ejcterior of the centrum appears formed by smooth compact bone. 



In the osseous fishes I find that the centrum is usually ossified from six 

 points, four of which commence, as Rathke || describes, in the bases of the 



* Brandt & Ratzeburg, Medizinische Zoologie, 4to, 1833, t. ii. pi. iv. fig. 1. 



t Hunterian Lectures on Vertebrata, 1846, p. 53, fig. 12. 



t See Agassiz, Recherches sur les Poiss. Foss. t. iii. pp. 361, 369. 



§ Hunterian Lectures on Vertebrata, 1836, p. 55, fig. 13. 



Ii Abhandlungen zur Bildungs und Entwickelungsgeschichte, Zweiter Theil, 1833, p. 41. 



