order v THEROMORPHA 187 



symphysis. The femur and cms are somewhat longer than the corresponding 

 bones of the fore-limb. 



Dicynodon, Owen (Figs. 291, 292). Skull attaining a length of 0'5 m. 

 Transition from parieto-frontal to nasal region gradual ; maxillae with a pair 

 of tusk-like teeth. Abundant and represented by over a dozen species in the 

 Karoo Formation of South Africa. 



Oudenodon, Owen. Like the preceding, but toothless ; possibly referable 

 to females of Dicynodon. 



Ptychognathus, Owen (Lystrosaurus, Cope), (Fig. 290). Frontal and nasal 

 regions sloping at a sharp angle from top of the skull. Smaller than 

 Dicynodon, which it accompanies. 



Gordonia, Newton. Form of skull similar to Dicynodon, but much smaller, 

 with more delicate bones and diminutive teeth ; premaxilla nearly vertical. 

 Supposed Trias of Elgin, Scotland. 



Geikia, Newton. Resembling Ptychognathus, but toothless. Elgin Trias. 



Eurycarpus, Seeley (Fig. 293); Keirognathus, Seeley ; Theriognathus, Titano- 

 suchus, Platypodosaurus (Fig. 294), Owen. All founded on fragmentary remains 

 from the Karoo Formation of South Africa. 



Sub-Order 4. PLACODONTIA. von Meyer. 1 



Bones of temporal region forming a single broad arcade ; supratemporal vacuity 



large, opening superiorly; orbits and external nares laterally placed; the latter 

 separate and remote. Palate with a paired longitudinal series of pavement teeth, and 

 similar ones present in single series on the dentary. Anterior teeth above and below 

 eylindro-conical ; maxillae with a series of depressed, conical, or molariform teeth. 



Placodonts are remarkable for their peculiarly modified dentition, which is 

 unparalleled among reptiles. The pavement-like crushing teeth of the palate 

 and lower jaw attain considerable size, and exhibit flat or slightly arched 

 crowns, usually dark- coloured, and smooth or finely wrinkled. They are 

 replaced by successional teeth developed beneath them. 



The skull is similar in form to that of Dicynodonts and certain Therio 

 donts ; and as in these groups the temporal and jugal arches unite to form a 

 single, broad, bony arch across the postero-lateral region of the cranium. The 

 quadrate is fused with the squamosal and jugal, and exposes a prominent 

 transverse condyle. On the under side the palatines and pterygoids unite to 

 form an extensive bony palate for the support of the pavement teeth. The 

 internal nares are placed far forwards, but the external openings and orbits 

 occupy similar positions to those in Dkynodon. Of the rest of the skeleton 

 nothing is known. The detached teeth were for a long time confounded with 

 fishes, until their reptilian nature was demonstrated by Owen. All the 

 remains are from the marine Trias of central Europe. 



Placodus, Agassiz (Figs. 295, 296). Skull not much longer than broad, 

 superiorly arched, the snout somewhat produced. Premaxillae and symphysis 

 of lower jaw with eylindro-conical incisors. On either side of the palate and 



1 Literature : 



Meyer, II. von, Unterkiefer vou Placodus andriani (Palaeontogr. vol. X. p. 59), 1862. — Ibid. vol. 

 XI. 1863.— Monster, G. von, Ueber eiuige ausgezeichnete fossile Fischzalme aus dem Muschelkalk 

 bei Bayreuth. 1830. — Ueber Placodus rostratus (Beitrage zur Petrefaktenkunde, pt. 4), 1843. — 

 Oioen, 11. , Description of the skull and teeth of Placodus laticeps (Phil. Trans, vol. XLIX.), 1858. 



