CHAPTER V. 

 (II. ) GN ATHOSTOM AT A. 



THE mouth is supported by definite jaws. 



ICHTHYOPSIDA. 



The epiblastic exoskeleton is generally unimportant, the 

 mesoblastic exoskeleton is usually well developed. 



The notochord with its membranous sheath (1) may remain 

 unmodified, or (2) may be replaced by bone or cartilage derived 

 from the skeletogenous layer, or (3) may be calcified to a 

 varying extent. 



The first vertebra is not homologous throughout the whole 

 series and so is not strictly comparable to the atlas of Saur- 

 opsids and Mammals. 



The centra of the vertebrae have no epiphyses. The 

 skull may be (a) incomplete and membranous, or (6) more or 

 less cartilaginous, or (c) bony. Membrane bones are not 

 included in the cranial walls, and there are large unossified 

 tracts in the skull. When membrane bones are developed in 

 connection with the skull, a large parasphenoid occurs. The 

 basisphenoid is always small or absent. The skull may be 

 immovably fixed to the vertebral column, or may articulate 

 with it by a single or double occipital condyle. When the 

 occipital condyle is double, it is formed by the exoccipitals, 

 and the basi- occipital is small or unossified. The mandible 

 may be (a) cartilaginous, (b) partially ossified, or (c) membrane 

 bones may be developed in connection with it, if so, there is 



