SKULL — TELEOSTOMES 



93 



Fig. 98. — Base of cranium of 

 Pleuronectes (Cole and Johnstone, 

 '02). eo, exoccipital; epo, epiotic; 

 op, opisthotic; pto, pterotic; so, 

 supraoccipital. 



of the cranium (tig. 98) are the four occipitals, the supraoccipital 

 being the least constant, and when present, it may be excluded from 

 the margin of the foramen magnum by the meeting of the exoccip- 

 itals above the opening. The supraoccipital usually develops a 

 strong (occipital) spine, pointing backwards, in response to the 

 strain of the large dorsal trunk muscles. 

 The cranial floor largely persists as car- 

 tilage, there being a large parasphenoid 

 below it, but sometimes there is a small 

 basisphenoid behind the hypophysis. 

 More constant, and extending up in the 

 side walls of the skull are the ali- and 

 orbitosphenoids of either side, these 

 being well developed in platybasic fishes, 

 but are reduced (especially the orbito- 

 sphenoid — lacking in many Acantho- 

 pteri) where an interorbital septum 

 occurs. 



Otic bones are more numerous in 

 most Teleostomes than in higher 



Vertebrates. When best developed, an opisthotic (very variable, 

 sometimes several, sometimes lacking) ossifies in the posterior cupula. 

 The prootic, in the floor and anterior wall, extends into the orbital 

 region, the fifth nerve passing in front of or through its anterior part. 

 The sphenotic, usually closely associated with the overlying post- 

 frontal, arises in the anterior cupula, and on the posterior outer 

 surface of the capsule is a compound squamoso-pterotic, its tw^o 

 elements distinct in the early stages. In the angle between supra- 

 and exoccipital, and pterotic there may be an epiotic (paroccipital) 

 forming the postero-lateral angle of the cranium with which the 

 supracleithrum of the shoulder girdle articulates. 



The dermal bones vary. Usually in the cranial roof are parietals, 

 frontals and nasals, the bones of each primitive pair frequently 

 fusing, while parietal and frontal may unite as a fronto-parietal. 

 The supraoccipital (the interparietal often fused with it) is sometimes 

 visible behind the parietals, occasionally extending forwards and 

 separating them. At times the supraoccipital does not ossify and 

 then the cranium may end with a transverse row of dermal bones, 

 the middle two of which (dermoccipitals) are often called supraoccip- 



