30 PKOF. G. B. HOWES A^D MK. H. H. SW1NNERTON ON THE 



both the centrum and the arch. Giinther describes the heads of the ribs as " low and 

 compressed," and attributes to them a wholly central articulation. Baur has more 

 correctly shown (86. p. 735) that the connection is rather with the arch, but he never- 

 theless insists that " in reality the rib never completely forsakes the centrum." 



We can confirm the details of Baur's description and extend them to Stage S. 



Of the so-called " lumbar " ribs, i. e., the posterior two to four of the praesacral 

 series, the head of the last one may show increased expansion, and in this way that 

 may become in a sense transitional to the sacral rib, the special feature of which 

 (PI. II. fig. 5, r.s.) is its general robustness and greater attachment to the centrum. 

 Except to point out that the sacral ribs are ossified in the mariner of the prassacral 

 (PL II., cf. figs. 4 & 5), we have nothing to add tc recorded descriptions concerning 

 them. There are usually two pairs, of which the anterior are the more slender and 

 transversely disposed, the posterior the more robust and forwardly directed (PI. I. 

 fig. 12, r.s.). The iliac articulation is usually furnished by their united outer carti- 

 laginous extremities. 



Fig 6. 



Sacrum with hip-girdle of an adult Splienodon, bearing on the left side an abnormal set of ribs. 



c.a., articular cartilage ; i7., ilium ; is., ischium ; pb., pubis ; r.cd.\ first caudal rib ; 

 r.s. 1 , r.s. 2 , sacral ribs. Nat. size. 



In view of the frequent existence of three sacral ribs in many living Lacertilia ! , of 

 more than two in the Chelonia, of the occasional presence of a third in recent Cro- 

 codilia 2 , and particularly of that of at fewest three pairs in Palreohatteria 3 , interest 

 attaches to a skeleton preserved in the Dublin Museum of an adult Sphenodon (text- 

 fig. 6) having on the left side a trisegmental sacrum, on the right a sacrum and hip- 

 girdle normal in all respects. The third sacral rib on the left (r.cd. 1 ) is seen to be the 

 first caudal, enlarged and expanded. Externally it was in life continuous by its outer 



1 Stellio and Chamseleo, Credner, H. : op. tit. p. 505. Phrynocephalus, Cope, E. D. : Proc. Amer. Philos. 

 Soc. vol. xxx 1892, p. 207. For other examples cf. Siebenrock, F. : Ann. naturhist. Hofmus. Wien, Bd. vii. 

 p. 373, and Sitzb. Akad.Wiss. Wien, Bd. civ. p. 67. 



2 Baur, G. : Zoolog. Anz. Bd. xii. 1889, p. 240. 



3 Credner, H. op. cit. p. 505. 



