DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 37 



and Sauranodontidae), characterized by the triserial condition of the " abdominal 

 ribs," and the lower or Proterosauria (Palaeohatteriidae, Proterosauridae, and Champso- 

 sauridse), in which their segmentation is multiserial. The adult Sphenodon, if 

 classified by its " plastron," can only be referred to the former the developing 

 Sphenodon might with justice be relegated to the latter and lower suborder ! In this, 

 Boulenger's far-sighted classification receives welcome support. 



Moreover, the multisegmented condition of the " plastron " segments is nothing 

 short of a Stegocephalian character. 



The Skull and Visceral Arches. 



In compliance with passing custom, we have adopted in this section of our work the 

 reconstructional method, as before stated (antea, p. 7). 



Beyond the memoirs of the late Kitchen Parker, and some recent preliminary notes 

 by Gaupp, together with the observations of Ley dig (op. cit.), Born, and Hofmann, duly 

 mentioned by him, little has been written upon the actual development of the 

 Lacertilian chondrocranium 1 ; and since in the absence of figures we find it difficult 

 to follow the details of Gaupp's descriptions, we deem it more prudent to describe 

 connectedly the processes taking place in Sphenodon, which are simple and straight- 

 forward, and highly instructive in themselves. 



The Chondrocranium. In Sphenodon the cartilaginous elements of the skull arise 

 before those of the greater part of the vertebral column, and the ossifications in 

 membrane before those in cartilage of both skull and vertebral column. The first 

 differentiation to form the primordial cranium which we have observed is at Stage P, 

 and it consists mainly of pro-cartilage. It can be resolved into two perfectly distinct 

 portions, excluding the mandibular arch an anterior common to the olfactory and the 

 trabecular regions, a posterior involving the sphenoccipital and auditory regions 2 . 

 Dealing with these individually, the anterior portion is seen to embrace the trabeculae 

 (PI. III. figs. 1 & 2, tr.) and an ethmoidal constituent, consisting of an extensive 

 basal plate with two pairs of outgrowths an anterior or olfactory pair (n.e.) and a 

 posterior pair to be hereafter termed the ethmosphenoidal plates (e.s.), The trabeculse 

 (tr.\ widely separated and enclosing a spacious pituitary foramen (py. 1 ), are already 

 hyaline, and pass gradually into the lateral edges of the basal ethmoid. The 



1 Gaupp, E. : Verhandlg. anat. Gesellsch. Sammlg. v. (Anat. Anz. Bd. iii., Suppl.) pp. 114, 120 (1891), also 

 ibid. Sammlg. xii. (Anat. Anz. Bd. xiv., Suppl.) pp. 157-163 (1898), and Ber. naturf. Gesellsch. Freiburg i. B. 

 Bd. x. pp. 302-316 (1898). 



- Left uncoloured in our illustrations. Concerning the embryo which furnished the sections for figs. 1 

 to 3, PI. III., it is regrettable that before investigating it we had handed it on to another, who, in 

 decapitating it for other use, had cut through the auditory region. We found, however, on examination 

 of slightly younger specimens, evidence sufficient for the extension of the differentiation into pro-cartilage 

 into the regions included in the dotted lines in the figs. 



