16 SALMONID^ OF BRITAIN. 



As already remarked the number of vertebrae in the spinal column _ of 

 Salmonoids is not invariably identical in every individual of any given species, 

 and this inconstancy may be natural or consequent upon accident or disease. 

 Occasionally a very large one may be seen occupying the space where two naturally 

 exist, as shown by the number of neural or haemal arches and spines (figure 7) 

 for two bones being present, or even the bodies of several vertebrae appear to coalesce 

 into one as will be more fully observed upon when the description of the hog- 

 backed-trout is arrived at. On the other hand two or even three small vertebras 

 may take the place of one large one. In fishes wherein the number of these 

 bones is normally about sixty an extreme variation of four or five, especially when 

 such has not been shown to be constant, cannot be looked upon as beyond what 

 might reasonably be anticipated in a single species, and inconstant variations, 

 unless conjoined to other difierences, would scarcely justify the creation of new 

 specific names for their reception. 



The skull, or that portion of the skeleton which articulates posteriorly with 

 the first vertebra, encloses the brain as well as forms the face, and although it 

 contains too many component parts to be fully discussed here, still a few remarks are 

 necessary respecting such bones as more or less constitute the orbit, mouth, and 

 respiratory organs, while it must be noted that the bones, especially of the head, 

 undergo great changes with age, even after the maturity of the fish has been 

 arrived at. 



The upper jaw is formed of two main bones, the premaxillaries (Plate 1, No. 

 17), which are furnished with teeth and placed at the anterior end of the snout in 

 the middle line of the head. The posterior portion of the bone passes upwards and 

 backwards, while the anterior or the tooth-bearing portion forms the upper and 

 front edge of the mouth. The maxilla (18) is likewise toothed and articulates 

 with the palatine bone : it is somewhat dilated towards its distal end, where a 

 small, ovoid-shaped, supplementary bone (18') rests upon its upper and outer 

 surface and has been likened to an ossified labial cartilage. The maxilla and 

 premaxillary are connected together by a narrow membrane, causing them to 

 move parallel to each other. The width and strength of the maxilla in adult 

 fishes depends to a considerable extent upon the food on which the fish has sub- 

 sisted during its lifetime, it being comparatively stronger in the trout than in the 

 salmon and in fresh-water than in anadromous races. If it has been n3.uch called 

 into action it may greatly exceed in size and strength what is present in examples 

 in which it has been less employed. It appears to be almost the invariable 

 rule that the jaws are larger in adult males than in females, while in old 

 examples the lower jaw is provided with a hook at its extremity, that is seasonably 

 developed during the breeding period, but is always more or less present in 

 old specimens. 



The mandible or lower jaw, consists of two branches or rami, connected 

 together anteriorly in the median line by ligament, where it is called the symphysis 

 and which union becomes ossified in old fish. Each branch of the jaw is made up 

 of several pieces in the young, which are solidified into one by age. The upper 

 and anterior being the largest portion of the jaw (34), bears teeth and is 

 termed the dentary part, its inner surface is deeply excavated in order to receive 

 a cylindrical cartilage or "Meckels cartilage," the remains of an embryonic 

 condition, the small angular (36) and articular (35) portions of this bone being 

 ossified portions of that cartilage. Behind the dentary portion and along the 

 upper edge is the articular bone which on its upper and posterior edge has an 

 articular cavity which receives the quadrate (26) and also sends up a coronoid 

 process to which a ligament from the maxillary and masticatory muscles is 

 attached : this bone anteriorly is more or less sheathed in the rougher dentary 

 portion. 



The suborbital ring of bones (19) passes round the lower edge of the eye, and 

 consists of several pieces, the anterior of which is often termed the pre-orbital or 

 lachrymal. 



The opercular pieces or those bones forming the gill-covers (28, 30, 82, 33) 

 have already been referred to (page 12 ante) the most anterior or innermost the 



