Xvi SKELETON. 



processes of the vertebrae are more soldered to the bodies, and not so well 

 developed. 



The skull, or' that portion of the skeleton which is situated at the front 

 end of the body articulating posteriorly with the first vertebra, is destined 

 to enclose the brain as well as form the face. It varies much in form in 

 different families of fish, and contains too many component parts to be fully 

 discussed here, but a few remarks are necessary respecting such as more- or 

 less enter into the orbits, mouth and respiratory apparatus. 



The suborbital ring of bones, or those which pass round the lower edge of 

 the eye, consist of several pieces, the anterior of which is generally the largest, 

 and termed the pre-orbital or lachrymal (fig. 3, I). 



The mouth is formed on several plans. At the front edge of the upper 

 jaw in the middle line is a tooth-bearing bone, the premaxillary (fig. 3, p. m), 

 also termed the intermaxillary, which is continued backwards in the form 

 of a" flat process, which in some forms even extends so far as the middle of 

 the orbit. The premaxillary rests against the toothless maxillary (fig. 3, m), 

 with which it is- in opposition, and the two move together. There are many 

 modifications of this form of osseous structure of the upper jaw. Thus in 

 the salmon the dental portion of the premaxillaries is comparatively decreased 

 in size, and the maxillary likewise bears teeth. 



The lower jaw or mandible consists of two branches or rami, one on 

 either side connected together anterioi-ly in the middle line by a ligament, 

 this portion being termed the symphysis. Each ramus consists of several 

 pieces, the largest which bears teeth being termed the dentary (fig. 3, d), 

 while a small one at the hind end of each is termed the articular and 

 articulates with the quadrate bone (fig. 3, q). 



The hyoid arch is attached to the temporal bones by two slender styliform 

 ones termed stylo-hyoids; this hyoid arch being composed of a central and 

 two lateral portions. The bones'along the two branches commencing from 

 behind forwards are the " epihyal " to which the stylo-hyoids are attached : 

 the ceratohyal to which the branch'iostegal rays are attached at their inner 

 end, then two small bones termed basihyals, between which the small glos- 

 sohyal or os Unguis extends forwards to the tongue, while a single bone, 

 the urohyal, passes backwards. This last bone in some fishes extends some 

 way even to the union of the coracoid bones, thus constituting an isthmus, 

 separating the two branchial openings. 



The five branchial arches, four of which bear gills (see page xlyii) and 

 one is destitute of them, are externally bounded by the hyoid arch ; while 

 inferiorly and along the median line they are attached to a chain of bones, 

 the basibranchials, which are situated above the urohyal and are anteriorly 

 connected with the body of the hyoid. These branchial arches pass 

 upwards, and are attached by ligaments to the under surface of the skull. 



