io8 A HISTORY OF FISHES 



manner of development of the several bones here, nor are 

 their technical names of great importance; for such details 

 reference must be made to any good text-book of zoology. 

 The chief point of interest lies in the later history of the bones 

 developed in connection with the primitive pterygo-quadrate 

 cartilage. These never function as an upper jaw, but, together 

 with certain dermal bones developing in the skin of that region, 

 form a bony palate or roof to the mouth, losing practically 

 every trace of their original nature. These palatine and ptery- 

 goid bones are firmly united with the cranium in front, and 

 behind are generally suspended by the hyomandibular bone, 

 the condition being essentially the same as in the Dog-fish. 

 The primitive lower jaw or Meckel's cartilage becomes entirely 

 invested or surrounded by bones, but retains its mobility as 

 well as its original function. To replace the primary one, an 

 entirely new upper jaw has arisen in Bony Fishes, made up 

 mainly of two bones on each side developed as dermal struc- 

 tures in the upper lip. These bones, which articulate with the 

 skull in front, are known as the praemaxillary and maxillary, 

 and the latter is sometimes provided with one or two small 

 bones attached to its upper edge, the supramaxillaries. In the 

 more generalised fishes the praemaxillaries are much shorter 

 than the maxillaries, which are provided with teeth and form 

 part of the border of the mouth (Fig. 46B). In many forms, 

 however, the praemaxillaries nearly or quite exclude the 

 toothless maxillaries from the gape, and the latter merely act 

 as a kind of lever for the protrusion of the former (Fig. 65). 

 The two praemaxillaries generally meet in the middle line, but 

 in some species {e.g. the Pike) they are well separated (Fig. 

 46c), and in the Eels (Apodes) they are altogether wanting. 



In a typical Bony Fish the mouth is placed at the end of the 

 head, and the upper and lower jaws are equal in length, but 

 this is by no means always the case, and considerable variations 

 both in size and position are found in certain fishes. In some 

 the mouth lies on the under side of the head, as in the Sharks, 

 its inferior position usually being due to the forward prolonga- 

 tion of the forepart of the head to form a rostrum. In the 

 Sturgeons [Acipenser), for example, the snout is particularly 

 massive, varying in shape and length in the different species, 

 and is provided with a transverse row of tentacles or barbels 

 on its lower surface (Fig. 45B) . The mouth itself is small and 

 circular, completely devoid of teeth in the adult fish, and is 

 capable of being protruded to a remarkable extent. The 



