7/6 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



SKELETON. 



PART I. Vertebral column and ribs of the adult. 



A typical vertebra from the trunk of Lcpidosteus has the 

 following characters (Plate 42, figs. 80 and 81). 



The centrum is slightly narrower in the middle than at its 

 two extremities. It articulates with adjacent vertebrae by a 

 convex face in front and a concave face behind, being thus, 

 according to Owen's nomenclature, opisthoccelous. It presents 

 on its under surface a well-marked longitudinal ridge, which in 

 many vertebrae is only united at its two extremities with the 

 main body of the vertebra. 



From the lateral borders of the centrum there project, at a 

 point slightly nearer the front than the hind end, a pair of pro- 

 minent haemal processes (/i.a.), to the ends of which are articu- 

 lated the ribs. These processes have a nearly horizontal direc- 

 tion in the greater part of the trunk, though bent downwards in 

 the tail. 



The neural arches (n.a.) have a somewhat complicated form. 

 They are mainly composed of two vertical plates, the breadth 

 of the basal parts of which is nearly as great as the length of 

 the vertebras, so that comparatively narrow spaces are left be- 

 tween the neural arches of successive vertebrae for the passage 

 of the spinal nerves. Some little way from its dorsal extremity 

 each neural arch sends a horizontal process inwards, which meets 

 its fellow and so forms a roof for the spinal canal. These pro- 

 cesses appear to be confined to the posterior parts of the ver- 

 tebrae, so that at the front ends of the vertebrae, and in the 

 spaces between them, the neural canal is without an osseous 

 roof. Above the level of this osseous roof there is a narrow- 

 passage, bounded laterally by the dorsal extremities of the 

 neural plates. This passage is mainly filled up by a series of 

 cartilaginous elements (Plate 42, figs. 80 and Si, i.e.) (probably 

 fibro-cartilage), which rest upon the roof of the neural canal. 

 Each clement is situated intervertebrally, its anterior end being 

 wedged in between the two dorsal processes of the neural arch 

 of the vertebra in front, and its posterior end extending for some 



