FAUNA OF THE GOSAU FORMATION. 



649 



the rib must have had a Y _s ^ a P e d form ; and just below the fork 

 of the Y on the posterior side is a moderately large vascular 

 foramen. 



The dorsal ribs are all imperfect at the distal end ; and some of them 

 have a much greater curve than others. The longest fragments are 

 imperfect, proximally and distally, and measure round the outside 

 curve about 11 inches, and across the chord a little over 10 inches. 

 But all the ribs agree in distinctive character, which is seen in side- 

 to-side compression along the proximal half of the visceral surface 

 and lateral expansion on the dorsal surface (PL XXVII. fig. 17), 

 so that in this half of the rib the transverse section is shaped 

 like a J (PL XXVII. fig. 18); but distally the rib expands 

 on the visceral surface, and the elevated ridge on the ante- 

 rior border disappears, the direction of the rib becomes some- 

 what oblique, and its section has a compressed ovate outline. One 

 consequence of this remarkable dorsal expansion is to form an ex- 

 panded table-like external surface which is convex in the direction 

 of the length of the rib, and nearly flat in the antero-posterior 

 direction ; so that the ribs closely resemble the combined rib and 

 costal plate of a Chelonian, and in the living animal the interspaces 

 between the ribs must have been as small as in many mammals, such 

 as the Buffalo and the Lesser Anteater. It is, of course, possible that 

 this expansion of the dorsal margin of the rib may be homologous 

 with the costal plates of Chelonians, since representatives of the 

 structure are also met with in Crocodiles, Uatteria, and birds. I 

 have no doubt that this table structure carried the heavy dermal ar- 

 mour with which these animals were weighted. One specimen (fig. 17), 

 which has the articular head preserved, has an interspace of 1^- inch 

 between the capitulum and tuberculum. The capitulum curves 

 slightly forward, is ±± inch in depth, and gradually widens on the 

 ventral surface to\ rds the articulation, where it is \ inch in dia- 

 meter. The tuberd im is relatively small, ^ inch from back to front, 

 and about \ inch if \ddth. It is somewhat reniform and rounded, 

 like the outside of a dne3 r . Immediately beyond the tubercle the 

 bone begins to widen , and the anterior ridge extends in the speci- 

 men 4 inches, while in other ribs it extends 6 inches, and in 

 some only 3. Its antero-posterior width is also variable, but, 

 where widest, in no specimen measures more than 1 inch, and is 

 usually about | inch. The compression of the rib just below the 

 tubercle gives a measurement of less than t 2 q inch, though in some 

 specimens it may be a little more and in others less ; while the depth 

 of the rib from dorsal to ventral surface is at first y 9 ^ inch, and be- 

 comes gradually reduced as the rib extends and loses its T -sna P e d 

 section. The side- to-side compression extends close under the ex- 

 ternal platform, so that both sides of the rib are concavely chan- 

 nelled. Several specimens show some amount of muscular roughness 

 on the transverse platform and a part of the rib distad to its termina- 

 tion ; and this is probably correlated with the muscular attachment of 

 dermal armour. The longest ribs preserved do not indicate a greater 

 length when complete than 14 inches. When they become obliquely 



