42 COMPARATIVE ANATOMY. 



and enamel may be, and often are, absent, but dentine 

 never. 



Developed within grooves npon surface of mucous mem- 

 brane, the primitive papillae may remain exposed, as in Sela- 

 chia (sharks), or each become encapsuled within mucous 

 membrane, as in many osseous fishes. When erupted, the 

 teeth are seen to be either attached to mucous membrane 

 alone, as in Pisces, or inserted in bony grooves (alveoli), which 

 may be continuous, as in Reptilia, or divided transversely into 

 sockets, as in Mammalia. 



Teeth vary greatly in numbers and position. As a rule, it 

 may be stated that animals developing large numbers of 

 teeth on surfaces remote from the region of lips, are inferior 

 to those erupting smaller numbers and restricting their site 

 to the anterior portion and sides of mouth. 



Teeth wanting in Aves, except in Archceopteryx. Rarely 

 absent in other vertebrates, as in Amphioxus (lancelet), Aci- 

 penser (sturgeon), Chelonia, Myrmecophaga (ant-eater), and 

 Manis (pangolin). 



Pisces. In cartilaginous fishes, teeth numerous, pointed, 

 tesselated. Teeth confined to jaws proper, as in Selachia 

 (sharks). In Osseous fishes teeth variable in number and 

 form. They may be conical, setiform, villiform, lobed, 

 curved, etc. Placed chiefly upon vomer, palatine, intermax- 

 illary, hyoid, pterygoid, inferior maxillary bones, and bran- 

 chial arches, they occasionally appear upon superior maxil- 

 lary, basi-occipital and nasal bones. Attachment in fishes 

 generally ligamentous, as in Selachia. Dentinal varieties, 

 vitrio-dentine, vaso-dentine, and osteo-dentine. Scarus (par- 

 rot fish) has apparently enamel and cementum in addition to 

 dentine. 



Batrachia. Teeth generally simple, as in Rana (frog). 

 Placed commonly on vomer, intermaxillary, and portions of 

 inferior maxillary bones. Remarkable form of, in Laby- 

 rinthodon. In tadpole, horny beaks are present. 



Reptilia. Ophidia, conical, placed on pterygoids, palatine, 

 maxillary bones. Teeth-like process on cervical vertebras in 

 Dtirodon. Peculiarity of construction of poison fang; the 



