18 NATURAL HISTORY OF VERTEBRATES. 



which disappear with the loss of the system of neuromastic canals. Such are, e. g., 

 the bones which surround the eye in most forms, and serve to protect the infraorbital 

 branch of the neuromastic canal system. 



It is now time to devote some attention to the cartilages of the jaws and their 

 products. The first arch early shows a division into two parts, the upper palato-quad- 

 rate, the lower mandibular. Between the two, the quadrato-mandibular articulation 

 is formed. The cartilages immediately entering into this joint are usually ossified as 

 quadrate above and articular below, but the fate of the remainder of the cartilages is 

 very different in different grouiDS. As many as five different ossifications in addition 

 to the articular may unite to form what we call the mandible or lower jaw. Some of 

 these may be cartilage bones, some membrane, and the greater part of the cartilage 

 may persist in some lower forms as the ' Meckelian ' cartilage surrounded by these 

 membrane bones. Of these the chief is certainly the dentary, the origin of which 

 from teeth is at once apparent, while others again, such as the coronoid and angular, 

 are developed in response to muscular strains. It is only in the fishes that the upper 

 part of this arch is so independent of the skull (Fig. 21). Generally the elements devel- 

 oped from it are connected with the proper cranial bones so intimately that they con- 

 tribute largely to the formation of the floor and side walls of the skull. In addition to 

 the quadrate, certain cartilage elements known as pterygoids and palatines appear in 

 this arch, and other membrane bones, chiefly of the category of cement bones, are 

 developed upon it. The latter are known as premaxillary, maxillary, jugal, and 

 quadratojugal. But the distinction between membrane and cartilage bones is here 

 again difllcult to maintain, for a palatine may be in part of cartilaginous origin and in 

 part a cement bone. The arrangement of the bones developed from the palato-quadrate 

 ai'ch is highly characteristic of the different gi'oups of vertebrates. Frequently two 

 distinct rows of bones are formed leading from the maxillary to the quadrate (Fig. 20), 

 the outer row including the jugal and quadrato-jugal, while the inner row is formed of 

 the palatine and pterygoid. Again, a transpalatine may connect the two rows in 

 many reptiles. The reason of the greater development of the i:)alatine and pterygoid 

 bones, in many higher forms, such as turtles and crocodiles, is to be sought for in the 

 separation of a nasal cavity from the primitive mouth cavity. Such a compartment of 

 the mouth cavity does not exist in fishes ; its formation is brought about by the devel- 

 opment of a partition, the palate, which shuts off a greater or less portion of the mouth 

 cavity as the nasal cavity. This partition may in part be destitute of an osseous 

 framework. It is then called the soft palate ; while the hard palate is formed by shelves 

 developed from the premaxillary and maxillary bones, with the assistance of the palatines 

 and pterygoids. The latter in mammals rarely contribute much to the formation of the 

 hard palate, being generally attached to the sphenoid bone as pterygoid processes. 



It is obvious from the above sketch, that many of the peculiarities of the skulls in 

 the various groups of vertebrates are traceable to the differing modifications of the 

 palato-quadrate arch. This is true of the relationship of the quadrate to the neigh- 

 boring bones. Sometimes it is very independent of the rest of the skull, as in snakes 

 and birds (Fig. 22) ; sometimes it is wedged firmly in with the neighboring bones, as 

 in turtles and crocodiles. 



In all Amphibia, Reptilia, and Aves, the suspension of the lower jaw is not effected, 

 as in fishes, by the aid of the following arch, but the quadrate alone articulates with 

 the mandible. "We shall consider presently the nature of the articulation of the lower 

 jaw in mammals. 



