690 



PROF. H. Gr. SEELEY ON THE EEPTILE 



bony union between these two parts of the skeleton, it was a union 

 established somewhat late in the animal's life. At this point the 

 bony plate is somewhat thinner than at its proximal fracture ; the 

 plate tapers apparently outwards, and is somewhat broken away 

 from the anterior proximal margin of the rib. The plate is hence, 

 as preserved, of a lanceolate form ; but its outline cannot be accu- 

 rately stated. 



The rib in this specimen differs from the ribs attached to the 

 costal plates of living Chelonians in being coextensive with the plate 

 — that is, as wide as the plate, and, as shown by the other specimen, 

 as long ; hence its limits are not defined distinctly in any part of 

 its extent, least so in the proximal part of the bone, where the sur- 

 face is convex from side to side. This convexity becomes narrower 

 and relatively more elevated as the bone proceeds outward, and is 

 confined to its anterior border ; where the rib terminates it is over 

 3 centim. wide, over 1 centim. thick, and, though compressed on 

 the anterior border, is more compressed on the posterior border. 

 The thickness of the combined rib and plate is about 2^ centim. ; 

 but even as preserved, the thickness at the fractured articular end is 

 7 centim. The underpart of the bone, which is slightly displaced by 

 a fracture consequent upon a minute dislocation, is convexly rounded 

 from side to side, and concave from within outwards, so as to pre- 

 sent a saddle-shape. The transverse width of the fragment pre- 

 served is about 6 1 centim. The anterior border is notched, as 

 though for the passage of an intervertebral nerve, or from the head 

 of the rib being free, as in ordinary Chelonia, though the conforma- 

 tion would rather suggest the former interpretation. There is one 

 curious character in evidence that the expanded lateral part of the 

 rib extended further towards the middle line than the articular 

 head, showing that the ribs had attained an unusual transverse de- 

 velopment consequent upon the expansion of the superimposed 

 plates. The transverse width of the head of the rib is 4 centim. ; 

 and here the fracture gives it an almost semicircular outline. 



The second specimen, already figured by Biinzel, pi. vi. figs. 4,5, has 

 a length, as preserved, of 48 centim. ; of this 39 centim. is occupied 

 by the blended rib and its superimposed dermal plate, the remaining 

 9 centim. consists of the extension of the plate alone beyond the head 

 of the rib towards the middle line of the body. The plate, where it is 

 presumed to have extended over the vertebra, is thin, and becomes 

 thinner the further it extends, both mesially and laterally. Its mar- 

 gins are imperfectly preserved, but have a wedge-shape. The costal 

 region wants a small portion in the middle of the plate in front ; 

 but throughout it has the anterior margin preserved rounded from 

 below upwards, and roughened in the proximal half, as though with 

 ligamentous attachment to the adjacent plate. On the upper surface 

 these lines at first run in a transverse direction, and then run for- 

 wards and inwards, just as on the under side they run downward 

 and outward. This oblique direction is strongly suggestive of the 

 oblique crossing of ordinary intercostal muscles. In length the 

 anterior margin of the bone is gently concave. The posterior margin 



