OWEN.—CLASSIFICATION OF REPTILIA. 77 
of the rib: the fore part is slightly produced at each angle into a zygapophysis 
looking upward and a little forward; the hinder part was much produced back- 
wards, 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 
eapsule 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. As such ventral or inferior cortical elements co-exist with seem- 
ingly complete centrums in the Ichthyosaurus,-thus affording ground for deeming 
them essentially distinct from a true centrum, the term “‘ hypapophyses ” had een 
proposed by Prof. Owen for such independent inferior ossifications in and from the 
notochordal capsule, and by this term may be signified the sub-notochordal plates 
in Archegosaurus, which co-exist with proper “hzemapophyses,” 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 inter- 
vertebral 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, co-existing there with 
the hemal arches, as in the menopome.* The hemal arches, which are at first 
open at their base, become closed by extension of ossification inwards from each 
produced angle, converting the notch intoa foramen. This formsa wide oval, the 
apex being produced into a long spine; but towards the end of the tail the spine 
becomes shortened, and the hzmal 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 that part; as the produced spines from both neural 
and hemal 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 skullof the Archegosaurus appears 
to have retained much of its primary cartilage internally, and ossification to have 
been chiefly active at the surface; where, as in the combined dermo-neural ossifi- 
cations of the skull in the sturgeons and salamandroid fishes, e.g., Polypterus, 
Amia, Lepidosteus, these ossifications have started from centres more numerous, 
than those of the true vertebral system in the skull of Saurian reptiles. The 
teeth are usually shed alternately. They consist of osteo-dentine, dentine, and 
cement, The first substance occupies the centre, the last covers the superficies of 
the tooth, but is introduced into its substance by many concentric folds extending 
along the basal half. These folds are indicated by fine longitudinal straight striz 
along that half of the crown. The section of the tooth at that part gives the 
*“ Principal Forms of the Skeleton,” Orr’s ‘Circle of the Sciences,’ p. 187, fig. 11. 
