CLASSIFICATION Of THE THERtUDOMTA. 67 



has small simple semicircular canals, and has a cochlea which 

 leaves no evidence of its existence on the bones. The fenestra 

 rotunda is represented by a notch on the ridge, which in the 

 bony skull separates the vestibular cavity from the vagal foramen, 

 and thus opens inside the brain-cavity. The stapes is always 

 very large and is perforated, the fenestra ovalis being a large 

 irregular hole. There is no groove for the external auditor}" 

 meatus. 



In Diademodoii the inner ear retains its original position low 

 down on the side of the brain-case and still shows only simple 

 semicircular canals. It has, however, a well-defined cochlea 

 housed in a crypt passing forward and inward, and curved 

 forward through about a quadrant of a circle. The fenestra 

 rotunda is a complete foramen, which opens indeed into the vagal 

 foramen, but does so on the outer surface of the neural cranium, 

 exactly as it does in the young Ornithorhynchus. 



The stapes, although still of good size, is much smaller than in 

 Pelycosaurs, and the fenestra ovalis is a neat round hole of 

 small size. The external auditory meatus is housed by a deep 

 groove. 



The nose of Diademodoii occupies a smaller space than that of 

 Dimetrodon, but the area, of its sensory epithelium seems to have 

 been increased by a great development of turhinal cartilage, now 

 only represented by a series of ridges, on the inner surface of 

 the nasals and prefrontals which once supported them. Nothing 

 of the kind occurs in Pelycosaui\s. 



Many other features in the nose of the Anomodonts can only 

 be discussed in connection with the septomaxilla, palate, etc., 

 and then only in a detailed discussion of individual forms. 



The Roof of the Skull. 



In Varanosaiorus the parietals are short, very broad, and with 

 the pineal foramen very far back. Their edges are separated 

 from the temporal fossae by a union of the postorbital and 

 squamosal. There is a large postfrontal lying on the roof of the 

 skull. The frontal is a large bone always entering into the 

 orbital margin. The prefrontals are large bones, almost equally 

 divided between the dorsal and lateral surfaces of the skull, 

 each bearing a depression on the outer face just in front of the 

 orbit. The nasals are narrow slips of bones. There is a small 

 supratemporal. 



In Diademodon the parietals are long, very narrow, and with 

 the pineal foramen between their anterior ends. They form the 

 inner margins of the temporal fossa? for a very long way. the 

 squamosal and postorbital being widely separated. There is no 

 postfrontal. The frontal is a small bone, not entering the 

 orbital margin . The prefrontals are small bones on the rounded 

 snout, with no depression in front of the orbit. The nasals are 

 wide, especially posteriorly. There is no supratemporal. 



5* 



