GEO UP B. 



Sub-Class TELEOSTOMI. 



On the head of the Teleostomi are usually to be noticed a pre- 

 ponderance of the paired cranial bones (a pineal foramen), and the 

 strengthening of the margins of the jaws with well -developed 

 tooth-bearing premaxillae and maxillae above, and dentaries below. 

 The outer, biting edges of the mouth and the main rows of functional 

 teeth are no longer supported by the palato-quadrate arch and 

 Meckel's cartilage ; the latter dwindles in importance anteriorly, 

 and the former becomes chiefly concerned in the roofing of the 

 palate. The cartilaginous cranium becomes very completely ossified, 

 not only by the development of endochondral bones in the deeper 

 parts, but also by the ingrowth from the surface of certain of the 

 dermal bones. It is sometimes difficult to decide to which of 

 these two categories some of the bones belong. Transition stages 

 from the dermal or membrane bone to the endochondral, and 

 perhaps also from the endochondral to the dermal, are frequently 

 found (p. 66). It also may happen that the main plate of a 

 given bone develops independently from that part which protects 

 the lateral line, the two often only fusing in the adult ; thus a 

 bone may acquire the appearance of having been originally formed 

 by the combination of two really distinct elements, whereas in 

 reality, as explained above (p. 222), the separation is secondary. 



The periotic capsule becomes invaded by the postfrontal 

 (sphenotic) from in front, and the pterotic (' squamosal ') from behind 

 (Figs. 237-8, 303-5). The former bone helps eventually to lodge the 

 anterior vertical semicircular canal of the ear, gradually encroaching 

 on the upper and anterior region of the auditory capsule. In the 

 more specialised fish (most Teleostei), the postfrontal abandons the 

 surface and sinks below, becoming a purely ' cartilage bone,' 

 except for the lateral-line element which may remain above. In 

 Amia the two portions are sometimes well developed and separate 

 (Allis [12]). Similarly, the pterotic invades the posterior upper 

 region of the auditory capsule, and lodges the horizontal semi- 

 circular canal. A bone known as the epiotic (external occipital) 

 develops on the upper posterior and inner region of the capsule, 



266 



