196 PALAEONTOLOGY. 



rib, pi ; the fore-part is slightly produced at each angle into 

 a zygapophysis looking upwards and a little forwards ; the 

 hinder part was much produced backwards, supporting two- 

 thirds of the neural spine, and each angle developed into a 

 zygapophysis, with a surface of opposite aspects to the anterior 

 one. In the capsule of the notochord three bony plates were 

 developed, one on the ventral surface, and one on each side, 

 at or near the back part of the diapophysis. These bony 

 plates may be termed cortical parts of the centrum, in the 

 same sense in which that term is applied to the element which 

 is called "body of the atlas" in man and Mammalia, and 

 " sub-vertebral wedge-bone" at the fore-part of the neck in 

 Enaliosauria. But as such neural or inferior cortical elements 

 co-exist with seemingly complete centrums in the Ichthyo- 

 saurus, affording ground for deeming them essentially distinct 

 from a true centrum, the term " hypopophysis" has been pro- 

 posed for such independent inferior ossifications in and from 

 the notochordal capsule ; and by that term may be signified 

 the sub-notochordal plates in Archegosaurus, which co-exist 

 with proper hsemapophyses (h) in the tail. In the trunk they 

 are flat, subquadrate, oblong bodies, with the angles rounded 

 off ; in the tail they bend upwards by the extension of the 

 ossification from the under to the side parts of the notochordal 

 capsule ; sometimes touching the lateral cortical plates. These 

 serve to strengthen the notochord and support the interverte- 

 bral nerve in its outward passage. The ribs (pi) are short, 

 almost straight, expanded and flattened at the ends, round 

 and slender at the middle. They are developed throughout 

 the trunk and along part of the tail, co-existing there with the 

 hsemal arches, as in the Menopome* The hsemal arches (h) 

 which are at first open at their base, become closed by exten- 

 sion of ossification inwards from each produced angle, con- 



* "Principal Forms of the Skeleton," Orr's Circle of the Sciences, p. 187, 

 fig. 11. 



