DEVELOPMENT OF THE SKELETON OF THE TUATARA. 37 
and Sauranodontide), characterized by the triserial condition of the ‘“ abdominal 
ribs,” and the lower or Proterosauria (Paleohatteriide, Proterosauride, and Champso- 
sauride), 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 Leydig (op. cit.), Born, and Hofmann, duly 
mentioned by him, little has been written upon the actual development of the 
Lacertilian chondrocranium!; 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 ?. 
Dealing with these individually, the anterior portion is seen to embrace the trabecule 
(Pl. III. figs. 1 & 2, ¢r.) and an ethmoidal constituent, consisting of an extensive 
basal plate with two pairs of outgrowths—an anterior or olfactory pair (7.e.) and a 
posterior pair—to be hereafter termed the ethmosphenoidal plates (e.s.), The trabecule 
(tr.), widely separated and enclosing a spacious pituitary foramen (py.’), are already 
hyaline, and pass gradually into the lateral edges of the basal ethmoid. The 
‘ Gaupp, E.: Verhandlg. anat. Gesellsch. Sammlg. y. (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. | 
to 3, Pl. 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. 
