TEETH OF VERTEBRATES. 401 



111 the lancelet there is no skull, or even the rudiments of 

 one (unless the semi-cartilaginous supports of the tentacles 

 be regarded as such), hence the Vertebrates are divided into 

 the skulless or acraniate (Acrania, represented by the lance- 

 let alone) and the skulled or craniate (Craniota}, the latter 

 series comprising all forms from the hag-fish to man. In 

 the Craniota the skulls may be, according to Gegenbaur, di- 

 vided into two groups. In the hag and lamprey the noto- 

 chord is continued into the base of a small cartilaginous 

 capsule, enclosing the brain, and which represents the skull 

 of higher Vertebrates (Craniota). This capsule behind is 

 continuous with the spinal column. 



With the skull of the second form two jaws are developed, 

 hence all the vertebrates above the hag and lamprey form a 

 series (Gnathostomata) opposed to the former, or Cyclos- 

 tomata. 



In the Gnatliostoinata there is a gradual modification and 

 perfection of the skull. In the sharks it may be quite sim- 

 ple and cartilaginous ; in the bony fishes it is highly special- 

 ized, consisting of a large number of separate bones. In 

 the Amphibians we first meet with a skull consisting of few 

 bones, easily comparable with those of mammals ; in the 

 reptiles and birds the ear-bones are external, forming the 

 large quadrate-bone by which the lower jaw is articulated to 

 the skull. A progress is seen in the mammals where the 

 quadrate-bone becomes internal one of the ear-bones (the 

 incus). Now also, the brain becoming much larger, evincing 

 a much higher grade of intellect, the skull is greatly en- 

 larged to accommodate the great increase in size of the 

 cerebrum and cerebellum, the perceptive and reasoning fac- 

 ulties predominating over those regions of the brain and 

 skull devoted to perceiving, grasping, and masticating the 

 food. 



Though not properly forming part of the skeleton or de- 

 veloped with it, we may here consider the teeth. 



The teeth of Vertebrates are formed from the modified 

 epidermis and cutis, or dermis ; the former secretes the 

 enamel and the latter is changed into the pulp or dentine. 

 The simplest form of tooth is conical. In the jawless hag 

 there are no teeth in the lips, but a single median tooth on 



