172 PALAEONTOLOGY 



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. 



As such neural or inferior cortical elements co-exist with 

 seemingly complete centrums in the Ichthyosaurus, thus 

 affording ground for deeming them essentially distinct from a 

 true centrum, the term "hypopophysis" has been proposed 

 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 heemapophyses (Ji) 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 ossifi- 

 cation 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 (j?t) 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 

 hamial arches, as in the Menopome.* The haemal arches (A), 

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

 sion of ossification inwards from each produced angle, con- 

 verting the notch into a foramen. This forms a wide oval, 

 the apex being produced into a long spine ; but towards the 

 end of the tail the spine becomes shortened, and the haemal 

 arch reduced to a mere flattened ring. 



The size of the canal for the protection of the caudal blood- 

 vessels indicates the powerful muscular actions of that part, 



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

 fig. 11. 



