DEVELOPMENT OF THE SKELETON OF THE TUATARA. 69 



Sphenodon, judged from our standpoint, must be regarded as the surviving represen- 

 tative of that group of animals ancestral to all the living Sauropsida, and to at least the 

 Dinosauria, Pterodactyla, and Ichthyosauria, of the past if not of the Mosasauria and 

 Dolichosauria also, and unquestionably intimately related to the Anomodontia. As 

 concerning the unmistakable Stegocephalian affinities of the Rhynchocephalia, it 

 becomes necessary to enquire more closely than hitherto into the sum of those 

 characters which constitute a Batrachian a Batrachian a Rhynchocephalian a Reptile 



Attention was in 1892 called by one of us (Journ. Anat. & Phys. vol. xxvi. p. 402) 

 to the fact that whereas no Batrachian living or extinct was then known to possess 

 more than two phalanges on each of its two innermost digits the formula for the 

 Class being 22342, that of the Sauropsida being 23454 (or something minus that by 

 reduction), it would seem impossible to derive the Sauropsidan condition from that 

 of any known Batrachian, not excluding the Stegocephalia. Recent investigation, 

 however, has modified this aspect of the question. Firstly, there has been discovered 

 an undoubted Stegocephalian ally having on the second digit of its fore limb three 

 phalanges and on the third four, and for its hind limb the formula 2 3 4 4 3 1 . And, 

 again, in Credner's Sclerocephaliis of the Permian, which, its massive quadrate notwith- 

 standing, he places among the Stegocephalia 2 , there existed a combination of the lowest 

 Stegocephalian type, amounting, in the structure of its dermal skeleton, almost to the 

 " Ganoidan," with a limb-skeleton (if the remains are rightly associated) of a veritable 

 lizard the phalangeal formula being 2 3 4 5 4 for the hind limb, and that clawed 3 . 



Again, as to the sternum, it has also been pointed out by one of us (' Nature,' 

 vol. xlviii. p. 269, 1891) that a sharp distinction may be drawn between the coraco-sternum 

 of the Batrachia (an archisternum] and the costal sternum of the Amniota (a neosternum], 

 the existence of which in any Batrachian or Stegocephalian has not been proved. 



And as to the skull, the now well-known fact that whereas in the Amniota the 

 hypoglossal nerve-bearing region, truncal in origin, is incorporated in the occiput, in 

 the Batrachia these nerves are postoccipital 4 , once again opens the gap between the 

 Batrachia and Amniota indeed, so markedly, that our ideas may be systematized by 



1 Ceraterpeton galvani, A. S. Woodward, Geol. Mag. (dec. 4), vol. iv. p. 297 (1897). 

 - Credner, H. : Zeitschr. deutsch. geolog. Gesellsch. Bd. xiv. 1893, p. 639. 



3 AVe are at a loss to understand the reason for the intercalation in the restored portions of the skeleton of 

 Pariasaurus in the British Museum of Natural History of a fourth phalanx to the second digit of the hind- 

 limh, especially as in the original description of the specimen it is the front-limb of which it is said (Phil. 

 Trans, vol. 183, B. p. 363) to be possible that one digit may have had four ! We fail to discover evidence of 

 more than three phalanges for any digit that is preserved. 



4 Restricting the terms to the Terrestrial Verrebrata, we leave aside the question how far the vagus-bearing 

 portion of the skull may be truncal also, and that of the undoubted parallelism which exists between the 

 Amniota and certain Ichthyopsida, concerning the union of skull and vertebral column (cf. the masterly 

 Memoir by Furbringer, M. : Gegenbaur Festschrift, Leipzig, 1897). 



