ON FOSSIL AND RECENT REPTILIA. 157 



and its capsule has connected it, by ligament, to the broad flat ossification of 

 expansions of the same capsule, forming the basi-occipital and basi-spheoid 

 plate. 



The vertebras of the trunk in the fully developed full-sized animal present 

 the following stage of ossification. The neurapophyses coalesce at the top to 

 form the arch, from the summit of which is developed a compressed, sub- 

 quadrate, moderately high, spine ; with the truncate, or slightly convex, 

 summit expanded in the fore-and-aft direction, so as to touch the contiguous 

 spines in the back ; the spines are distinct in the tail. The sides of the base 

 of the neural arch arc thickened and extended outwards into 'diapophyses' 

 having a convex articular surface for the attachment of the rib ; the fore part 

 is slightly produced at each angle into a zygapophysis looking upward and a 

 little forward ; the hinder partis much produced backwards, supporting two- 

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

 with a surface of opposite aspects to the anterior one. In the capsule of the 

 notochord three bony plates are 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' 

 iu Man and Mammalia, and 'sub-vertebral wedge-bone' at the fore part of 

 the neck in Enaliosauria. 



As such ventral or inferior cortical element co-exists with the separately 

 ossified centrum in certain vertebras of the Ichthyosaurus, thus affording 

 ground for deeming them essentially distinct from a true centrum, I have 

 applied the term ' hypapophysis' to 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 ' haem- 

 apophyses' in the tail. In the trunk the hypapophyses 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 intervertebral nerve in its 

 outward passage. 



The ribs 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, coexisting there with the haemal arches, as in the 

 Menopoma*. 



The haemal arches, which, at the beginning of the tail, are open at their 

 base, become closed, in succeeding vertebrae, by extension of ossification in- 

 wards from each produced angle, converting the notch into a foramen. This 

 forms a wide oval, the apex being produced into along spine ; but towards the 

 end of the tail the spine becomes shortened, and the haemal arch is 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 the long tail ; as the 

 produced spines, from both neural and haemal arches, bespeak the provision 

 made for muscular attachments, and the vertical development of the caudal 

 swimming organ. 



All these modifications of the vertebral column demonstrate the aquatic 

 habits of the Archegosaurus ; the limbs being, in like manner, modified as 

 fins, but so small and feeble as to leave the main part of the function of 

 swimming to be performed, as in fishes and Perennibranchiate Batrachia, by 

 the tail. 



The skull of the Archegosaurus appears to have retained much of its pri- 



* Principal Forms of the Skeleton, 'Orr's Circle of the Sciences,' p. 187, fig. 11. 



