PROF. SEELEY ON MAMMALS. 69 



Another chief difficulty in interpreting these extinct animals 

 arises first, from the diversity of the organic types included in the 

 Anomodontia, and secondly from the difficulty of defining the 

 terms mammal and reptile. The Anomodontia appear to show- 

 affinities with the lower limit of the reptiles as well as with more 

 than one type of mammal ; besides indicating some affinity with 

 the extinct reptilia. 



The form of the brain if it were available would be evidence of 

 affinity of some value. The lateral expansion of the cerebral hemi- 

 spheres and their extension backward over other parts of the brain are 

 mammalian features ; but they are not found in Anomodonts. The 

 brain cavity is imperfectly known, and there is no evidence that the 

 brain filled it. Its hinder part is deeper than wide at the base, its 

 sides are flat, and its superior border is a sharp ridge, in some 

 Dicynodonts. In form it is more like the mould of the brain cavity 

 in some Dinosaurs than the corresponding region in any living 

 reptile or mammal. One American ally of the African Theriodonts 

 has been shown to have the mould of the brain cavity compressed 

 from side to side, and like that of Belodon and some Dinosaurs. 

 Anomodonts are therefore not intermediate between reptiles and 

 mammals in form of the brain-cavity, any more than Dinosaurs 

 in this character are intermediate between reptiles and birds. 



The Monotreme mammals in being oviparous give a lower limit 

 for the mammalia, which invites comparison with Anomodonts, but 

 no Anomodont has shown pre-pubic bones or even such structures 

 as are found in marsupials and Ornithosaurs. On the other hand 

 the back of the skull shows a foramen above the articulation for the 

 lower jaw in Ornithorhynchus, which is situate in the same position 

 as the foramen which cuts into the quadrate bone in Dicynodonts, 

 in Hatteria, and other animals. Thus the articulation with the lower 

 jaw is made in the same way in both, and presumably by the 

 quadrate bone. Owen did not recognise the quadrate bone in 

 Ornithorhynchus only because he believed that bone to be the 

 tympanic. He figured a skull which shows many distinct sutures, 

 some of which are reptilian divisions of the skull. One skull in the 

 Royal College of Surgeons' Museum shows pre-frontal and post- 

 frontal bones, and indication of separation of the bone, in the place 

 of the supra-temporal, which is external to the quadrate bone. 

 These bones, hitherto characteristic of reptiles, are thus common 

 to Monotremes and Anomodonts, and comparable. 



The Theriodont division of the Anomodonts with teeth of 

 mammalian type, want the foramen above the condyle for the 

 lower jaw ; the quadrate bone is very small, and the articular 

 region for the lower jaw is like that in higher mammals. It is 

 a distinct type from the Dicynodont group. Thus the Anomodonts 

 have affinities with at least tw^o chief divisions of the mammalia. 



The Theriodonts also have affinities with the Labyrinthodont 

 reptiles, in the presence of intercentra in the vertebrae, and bones in 



