714 COMPARATIVE ANATOMY 



it is called a digitigrade foot, while, if the animal, such as the horse or 

 cow, walks upon hoofs which are homologous to the nails on the hands 

 and feet of higher forms, such a foot is called an unguligrade foot. 



There may be variations and fusions in all these animals. For ex- 

 ample, in the horse, it is only the third digit which persists in a func- 

 tional condition. 



SUMMARY OF THE CRANIUM 

 CYCLOSTOMATA 



The cranium lies entirely beneath the brain, and forms neither side 

 nor roof for the latter (Fig. 422). The cranial cartilages are sometimes 

 said to be homologous with those of the embryonic fish skull. 



DOGFISH 



The investing bones are closely applied to the roof and floor of the 

 chondrocranium, and modify its form considerably by projecting beyond 

 the cartilaginous part, and concealing apertures and cavities (Fig. 407). 

 The large frontals cover the greater part of the roof of the skull, con- 

 cealing the fontanelles and furnishing roofs to the orbits. Immediately 

 behind the frontals is a pair of very small parietals; in front of them 

 is an unpaired supra-ethmoid, to the sides of which are attached a pair 

 of small nasals. On the ventral surface is the large parasphenoid, which 

 forms a kind of clamp to the whole cartilaginous skull-floor ; and in front 

 of, and below, the parasphenoid is the toothed vomer. Encircling the 

 orbit is a ring of scale-like bones, the sub-orbitals. 



PISCES 



The fish skull (Fig. 409) is covered above and below by numerous 

 dermal investment bones which are much like those of the primitive 

 extinct amphibia Stegocephali. By boiling, all the investment bones 

 are loosened and when removed a chondrocranium like that of the dog- 

 fish is seen. 



In fishes there are primary and secondary structures in the jaw as 

 in the cranium. The primary upper jaw (palatoquadrate) is considered 

 homologous with the upper jaw of the dogfish. It does not, however, 

 remain cartilaginous but is ossified by five replacing bones : the toothed 

 palatine in front which articulates with the olfactory capsule; the ptery- 

 goid on the ventral edge; the mesopterygoids on the dorsal edge of the 

 original cartilaginous bar, and the quadrate at the posterior end of the 

 latter. These bones do not enter into the gap, and consequently do not 

 constitute the actual upper jaw of the adult fish. External to them are 

 two large investing bones, the premaxilla and the maxilla, which to- 

 gether, form the actual or secondary upper jaw. They both bear teeth. 



