LTASSIC FORMATIONS. 89 



a few feeble ridges at its peripheral and commonly lower third part. It is rare to find this 

 element unanchylosed with the succeeding centrum (ib., fig. 5, c x). The neural surface of 

 the atlantal centrum (ib., fig. 3) is divided equally between the medial quadrate tract 

 ( m ) for the myelon, 1 and the lateral subtriangular depressions (« p ) for the neurapophyses. 

 The length or fore-and-aft diameter of the atlantal centrum is usually relatively less than 

 in the trunk- or tail-vertebrae of the same individual. The neurapophysial surface bends 

 down upon the side of the centrum, forming a prominence (fig. 5, d) on its upper part, 

 representing the diapophysis ; beneath this, with a non-articular interval, projects a low 

 obtuse parapophysis (ib., p). 



The neurapophyses of the atlas (fig. G, «), as far as I have been able to infer from this 

 commonly mutilated or much disturbed part of the fossil skeletons, were not united 

 together atop, or there developed into an exogenous spine, but retained their distinct- 

 ness, like their antecedent homotypes the exoccipitals (PI. XXVI, fig. 1, 2). In the best 

 preserved specimen each atlantal neurapophysis is bent back at the middle of its length, 

 the upper compressed portion overlapping the fore part of the base of the neural spine 

 of the axis, as shown in PL XXIII, fig. 6. 



Of the existence of the atlantal pleurapophysis (PI. XXIII, fig. 5, pi, a), each being 

 joined by a bifurcate proximal end to the di- and par-apophyses of the centrum, there is 

 better evidence. Such rib was short, directed outward and backward ; and is uncon- 

 nected, distally, with any hsemapophysis (PL XXI, fig. 1, pi). 



The hypapophysis of the atlas (PL XXIII, fig. 5, hy, a, and PL XXVII, fig. 2) is an 

 irregular triangular robust ossicle, smooth and convex on its inferior and free surface, with 

 the opposite articular surface divided into three facets. The anterior of these (ib., fig. 



2, a) is smooth and concave, completing, with the concave part of the atlantal centrum, 

 the cup for the basioccipital ball ; an almost flat rough tract (ib., b) next below articulates 

 synchondrosaly with the corresponding rough surface of its centrum. Beneath this is a 

 smaller flat roughish surface (ib., c ), sloping backward from the one above, for articulation 

 with the succeeding hypapophysis. Such are the complex characters of the first trunk- 

 vertebra of Ichthyosaurus. 



The centrum of the second vertebra (PL XXIII, fig. 5, ex, and PL XXVII, figs. 



3, 4, 5), has a flattened, roughish, anterior surface (fig. 4), like the posterior one of the 

 first vertebra, with which it sooner or later coalesces. The hind surface of the axis- 

 centrum (fig. 5) is more deeply and entirely concave, with a sharpish circumferential 

 margin. On the upper surface of the centrum (fig. 3) the myelonal surface (m) is similar 

 in size and shape to that of the atlas, but is shallower. The neurapophysial surfaces 

 ( n , n) are less excavated, and the diapophysial productions (fig. 4, d, d) upon the sides of 

 the centrum are more prominent, better defined as processes. The same may be said of 

 the parapophyses (ib., p,p), which project close to the fore border of the side surface and 

 show a more distinct facet for the head of the axial rib than do those of the atlas. The 



1 ' Spinal marrow,' ' spinal cord,' of ' Anthropotomy.' 



