94 COMPARATIVE ANATOMY 



quadrate bar, which remains separate from its fellow and is con- 

 nected with the skull-base anteriorly. In connection with the 

 anterior part of this bar the palatine (investing and replacing 

 bone) is formed, and with the posterior part a quadrate. 

 Between these, bony elements are developed which are known as 

 pterygoids, of which may be distinguished a replacing meta- 

 pterygoid, an entopterygoid, and an investing mesopterygoid or 

 ectopterygoid (cf. pp. 82-84). These bones are already represented 

 in Bony Ganoids, and form, together with the base of the skull, 

 the roof of the oral cavity. 



The olfactory sacs are sunk in the ethmoid cartilage, in which 

 region supraethmoid and lateral ethmoid (ectethmoid) bones are 

 developed. 



In the auditory region, as in Bony Ganoids, are a prootic, an 

 epiotic, and an opisthotic, the most important of which is the 

 prootic. The opisthotic usually does not form an actual part of 

 the auditory capsule, with which, however, as already mentioned, 

 other bones (pterotic, sphenotic) may come into relation. 



In the occipital region, with which vertebral elements are 

 assimilated, are exoccipitals, which largely or entirely surround the 

 occipital foramen, and a basioccipital, as in Bony Ganoids, as well 

 as a very variable supraoccipital, which is wanting in the last- 

 mentioned group (Fig. 68, B). Where the basioccipital is in contact 

 with the vertebral column, it presents a concavity containing 

 notochordal tissue. 1 



Forming the margin of the upper jaw are a premaxilla and a 

 maxilla. These play an important part in all Vertebrates from 

 the Bony Ganoids onwards, but in Teleosts more particularly they 

 show considerable variation with regard to their relative develop- 

 ment, form, and arrangement, and in many cases the maxilla takes 

 no part in bounding the actual gape of the mouth, and does not 

 form a continuous bar with the premaxilla. Of the bones in relation 

 with the oral cavity, the vomer, the parasphenoid, the premaxilla, 

 arid the maxilla may bear teeth. The maxilla, however, is 

 edentulous except in the Physostomi. 2 



Besides the above-mentioned bones in connection with the 

 jaws, the cranial capsule of Teleosts is surrounded by other out- 

 works consisting of bony plates and bars. These arise as true 

 dermal bones in the region of the eyes (orbital ring), and in the 

 gill-covers (opercular bones) : the latter are similar in number and 

 name to those of many Bony Ganoids (p. 90). A large number of 



1 A curious asymmetry is seen in the head of adult Pleuronectidfe. When 

 hatched, these Fishes are quite symmetrical, but later on the eye of one side 

 becomes rotated, so that eventually both eyes are situated 011 the same side ; in 

 consequence of this, the skull also becomes asymmetrical. In many Teleosts a 

 canal, lying in the axis of the base of the skull, encloses the eye-muscles, and 

 opens on either side into the orbits. 



2 The tactile barbules present on the head of many Fishes, (e.g., Siluroids) 

 are supported by skeletal parts (cf. p. 82.) 



