ry5 STRUCTURE OF VERTEBRATES 



The auditory capsule forms a large part of the side wall of the 

 skull behind the orbit, and typically ossifies from three centres, 

 to form pro-otic in front, opisthotic behind and below, and epiotic 

 above. Behind these, floor, sides and roof of the skull are all formed 

 by a ring of bones, basioccipital below, supraoccipital above, 

 and an exoccipital on each side. 



In mammals the three otic bones fuse to form a single petrosal 

 or periotic, and in some, including man, this fuses with the 

 squamosal and tympanic (see below) to form the temporal ; in 

 marsupials and some others the presphenoid and orbitosphenoids 

 fuse with the mesethmoid to form the sphenethmoid. In addition 

 to these specific fusions there is a general tendency for sutures 

 to become obhterated with age. This tendency is even stronger 

 in birds, where hardly any separate bones are recognisable 

 in the skull of an adult animal. Modern amphibia have the 

 cartilage bones of the cranium much reduced in both number 

 and size. There is a single sphenethmoid (as in marsupials and 

 some extinct tetrapods), and in the ear region there are only 

 the pro-otics. The basisphenoid and the supra- and basioccipitals 

 have also been lost. 



In fishes the pterotic and sphenotic in the auditory capsule 

 are bones of mixed origin, i.e. partly cartilage and partly 

 membrane. This is also the condition of the prefrontal, a small 

 bone found above the nasal capsule in many groups. 



MEMBRANE BONES OF THE SKULL 



As we have seen above, the most primitive known verte- 

 brates had a complete bony covering to the head, and all 

 except the cartilaginous fishes and modern cyclostomes still 

 have some dermal bones which have sunk in from the surface. 

 There is good reason to believe that these were originally dermal 

 denticles or scales, for in some fish they consist not only of bone, 

 which is comparable to the dentine of the placoid scale, but also 

 of an outer layer of either ganoin or cosmine, the materials of 

 which the scales of bony fish are made. Whereas the scales of 

 bony fish (and of tetrapods) have lost their dentine, in the 

 dermal plates on the skull it is the other way about ; the ganoin 

 or cosmine has in general been lost, and the dentine has become 

 bone. 



In view of the origin of the dermal bones, it is not surprising 



