172 Transactions of the South African Philosophical Society. 
vertical plate of the premaxillary is situated the vomer, as an ir- 
regular triangular plate. It articulates with the premaxillary plate 
inferiorly by an interdigitating suture; but superiorly, the vomer 
divides into two delicate plates between which the premaxillary plate 
passes. Posteriorly the vomer articulates with the prespheroid 
clasping its anterior and inferior border. At its posterior and 
inferior angle the vomer divides to two small branches which pass 
outwards and backwards to meet the pterygoids. Above these 
branches the vomer for a short distance is quite separated from the 
prespheroid by an oval space. 
Though both the premaxillary and the vomer form such large 
median plates, the plates are for the most part exceedingly thin, 
as is well shown by Huxley’s sections. In figure 2 the letters AA, 
BB, and CC indicate approximately the positions of Huxley’s three 
sections. His front section, AA, lies slightly above the palatal plate 
of the premaxillary, and shows the internasal plate of the premaxil- 
lary to be moderately thick in its anterior region. His next section, 
BB, passes through the palatine bone in its anterior and greatly 
developed region. The median plate is here formed of the pre- 
maxillary above and the vomer below, the premaxillary fitting into 
a deep cleft in the vomer. In the posterior of Huxley’s sections the 
median plate is almost entirely formed by the vomer, only a very 
small portion of the premaxillary plate presenting above fitted into the 
cleft of the vomer. At its lower end the vomerine plate is seen to be 
cleft. Outside of the vomer the palatine and pterygoid are seen cut 
across—the lower being the pterygoid. 
On each side of the vomer near its posterior border is developed a 
small lateral wing which articulates with the palatine and forms with 
it the posterior walls of the nasal passage. In figure 2 a dotted line 
indicates the position of the lateral wings. 
The posterior part of the vomer is exceedingly well shown in 
more than one of the British Museum specimens, more especially in 
the imperfect skull which formed the type of Owen’s Ptychognathus 
boopis (Spec. No. 36253) and in the skull which formed the type of 
Cistecephalus chelydroides (Spec. No. 47068), and it seems remark- 
able that: there should have been any doubt as to its being the true 
vomer, more especially as in some mammals the posterior part of 
the vomer presents a strikingly similar appearance. Seeley,* in 
describing the skull of Dicynodon copet [= according to Lydekker, 
Ptychosiagum Murrayi, Huxley], suggests the possibility of this 
element being the vomer. He says: ‘In the constricted middle 
* H. G. Seeley, ‘‘ On Anomodont Reptilia and their Allies,” Phil. Trans., 1889, 
p- 241. 
