Il8 VERTEBRATE SKELETON 



Fusion of the quadrate to the cranium produces autostyly. A 

 bone (apparently dermal) extends from the quadrate forwards in 

 the roof of the mouth and is called 'palato-pterygoid/ the bones of 

 the two sides meeting in front and supporting a large tooth-plate 

 on either side, while farther forwards Ceratodus has a pair of 'vomer- 

 ine' teeth. A large parasphenoid extends from palatopterygoid 

 to about the hinder end of the skull. Meckel's cartilage persists in 

 the lower jaw and is bounded on the medial side by a large tooth- 

 bearing plate. Below is an angulare; no articulare is ossified. The 

 rest of the visceral skeleton is largely degenerate, the hyoid arch 

 being best preserved. In Ceratodus it has several parts; in the others 

 it is a single bone bearing a so-called interoperculum on its posterior 

 surface. 



A number of fossils are more or less closely related to the existing Dipnoi, 

 some differing but little from them, but others, frequently grouped as Arthro- 

 DiRA, are very dissimilar. This name alludes to the fact that the skull, composed 

 of a limited number of bones, is movably articulated with the armor plates of 

 the trunk, which may occur on both dorsal and ventral sides. Some genera, 

 like Coccostciis (fig. 124) have crania in which some homologies can be traced 

 with other Vertebrates, but other plates are uncertain. The giants, (Dinichthys, 

 fig. 123, Titanichihys) can be compared with Coccosteus, but not so readily with 

 modern Dipnoi. 



TETRAPODA. — The Tetrapod skull differs considerably from 

 that of fishes. With rare exceptions it is movable on the vertebral 

 column, the base of the cranium having one (most Sauropsida) or 

 two (Amphibia, Mammals) faces (occipital condyles) which articu- 

 late with the first (atlas) vertebra, a condition paralleled in fishes 

 only in a few Elasmobranchs. The pterygoquadrate apparatus is 

 more intimately connected with the cranium than is common in 

 fishes; the connexion may even be fusion of more or less of this carti- 

 lage, or of the bones (pterygoid and quadrate) which ossify in it, 

 with the cranium, so-called autostyly. Usually the pterygoid part 

 forms but a single bone, but some of the lower Tetrapoda have an 

 epipterygoid (columella cranii) arising from it. The palatine bone is 

 not connected with the pterygoid cartilage, but arises independently. 

 No Tetrapod has an operculum and all lack distinctly lateral-line 

 canal bones. 



Some of the most important differences between fishes and Tetra- 

 poda occur in connexion with the ears. Audition is well developed 



