xii. i 3 STEGOCEPHALIAN SKULL 327 



pattern of the bones on osteolepid and early amphibian skulls and to 

 confirm the remarkable similarity. The main new development found 

 in the skull of early amphibians was correlated with the modification 

 of the Eustachian tube in connexion with the sense of hearing, and 

 the need for a sensitive resonator to pick up the air vibrations. Already 

 in the earliest amphibians the opercular coverings of the gills were 

 lost (there was a small pre-opercular bone in *Ichthyostega) and the 

 spiracular opening thus uncovered acquired a tympanic membrane. 

 The hyomandibular cartilage, no longer concerned (if it ever had 

 been) with supporting the jaw, was modified to form the columella 

 auris, serving to carry vibrations across to the inner ear. At first, 

 however, there was no trace of the fenestra ovalis, the hole in the 

 auditory capsule into which the columella fits in the frog. In modern 

 urodeles the whole ear apparatus is much modified, there being no 

 tympanum. Instead the columella is fused to the squamosal and the 

 ear thus receives its vibrations from the ground. 



Other small changes in the skull in passing from the fish to the 

 amphibian stage include the increase in size of the lachrymal bone, 

 which also came to have a hole to carry the tear duct, draining the 

 orbit. A series of small bones surrounds the orbit in early amphibians, 

 as in fishes; large squamosals and quadrat ojugals support the quad- 

 rate. At the back of the skull these stegocephalians possessed various of 

 the small bones that are found in fishes but not in modern amphibians, 

 the supratemporal and intertemporal, post-parietal (much smaller 

 than in crossopterygians) and tabulars. In fact there are numerous 

 small bones, arranged in a pattern clearly recalling that of the fish 

 ancestor, but showing some reductions and less variation than in those 

 very variable fish skulls. This simplification (which was later carried 

 farther), together with some changes in the shape, are the chief trans- 

 formations that have converted the fish skull into the amphibian 

 skull. 



The palate of the early amphibians also resembled that of crosso- 

 pterygians, showing a complete plate made of vomer, palatines, 

 pterygoids, and ecto-pterygoids. These bones, as well as the pre- 

 maxillae and maxillae, often carried folded teeth (hence 'labyrin- 

 thodonts'), with a pit for a replacing tooth beside each one, an 

 arrangement similar to that of their fish ancestors (p. 270). The 

 internal nostril opened far forward, through the palate. The lower 

 jaw was covered by a number of dermal bones (Fig. 208), but the 

 actual jaw articulation was made between cartilage bones, the upper 

 quadrate, and the lower articular. 



