14 MAMMALIA class v 



locomotion, for running and grasping, for climbing and digging, for flying or 

 for swimming, so for each of these adaptations special modifications are 

 developed ; and as a result it often happens that animals of very distant 

 relationships exhibit similar structural modifications when subjected to 

 similar conditions of environment. These facts indicate a plasticity in the 

 entire organism, including the skeleton, which is clearly observable among the 

 various representatives of a group as they pass from one phylogenetic stage 

 to another in the course of geological history. The study of the changes in the 

 mammalian skeleton during its phyletic development, or so-called " kineto- 

 genesis," has been especially fostered by Cope and Osborn, whose guiding 

 principles are but slightly modified from the evolutionary A-iews of Lamarck.^ 

 More recently, however, H. F. Osborn has contested Lamarck's theory. 



With the study of the skeleton, that of the dentition of mammals is of 

 the greatest practical importance for systematic purposes, for not only do 

 the teeth show an extraordinary amount of variation, but they stand in the 

 closest relation to food habits and skeletal structure. The teeth have, there- 

 fore, from earliest times been especially valued for classificatory purposes, and 

 especially in Palaeontology, on account of their resistance to destructive 

 agencies during fossilisation.- 



The teeth (Fig. 17) of most mammals consist of enamel, dentine and 

 cement, and enclose a pulp cavity filled with cellular tissue, blood-vessels and 

 nerves. Vasodentine is found only among the Edentata. The cement 

 (crusta petrosa, c) forms as a rule only the thin crust over the root ; but 

 it may form a more or less thick crust round the crown, and sometimes 

 fills pits and valleys between the ridges on the top of the crown. The pulp 

 cavity (j)) occupies a large space in the crown and root or roots of young teeth. 

 In such young teeth the openings at the base of each root are wide, but as they 

 mature, these openings are greatly narrowed. When the teeth remain without 

 roots and the pulp gives rise to continuous growth, they are said to have 

 persistent pulps. Teeth with low crowns, well-developed and narrowed pulp 

 cavities, are termed hrachyodont ; while such as are of high cylindrical or 

 prismatic form, with an open pulp cavity, with or without roots, are called 

 hypsodont. 



The form of mammalian teeth is very largely determined by their 



' Literature : Cope, E. D., The Origin of the foot structures of tlie Ungulata. Jouni. Acad. Nat. Sci. 

 Philad., 1 874. — On the effect of impact and strains on the feet of Mammalia. Amer. Naturalist, 1881. 

 — The mechanical causes of the development of the hard parts of the Mammalia. Journ. Morph., 

 1889. — Primary factors of organic evolution, 1896. — Osborn, H. F., The evolution of the ungulate 

 foot. Trans. Amer. Phil. Soc, 1889. — Schlosser, M., ttber die Modiiikationeu des Extremitaten- 

 skeletes bei den einzeluen Siiugetierstamnien. Biol. Centralblatt, 1890. — Scott, W. B., On some 

 of the factors of the evolution of Mammalia. Journ. Morph., 1891. 



- Ameghino, F., Sur revolution des dents des mammiferes. Bol. Acad. Nacion. Cienc. Cordoba, 

 1896. — Recherches de morphologic phylogenetique sur les molaires superieui-es des ongules. Anal. 

 Mus. Nac. Buenos Aires, 1904. — Cope, E. D., The mechanical causes of the development of the 

 hard parts of the mammalia. Journ. Mori)h., 1889. — Flower, W. H., Remarks on the homologies 

 and notation of the teeth of the Mammalia. Journ. Anat. Physiol., 1869. — Koicalerski/, W., 

 Anthracotherium. Palaeontogr., 1874, Bd. XXII. — Osborn, H. F., Evolution of mammalian molars 

 to and from the tritul)ercular type. Amer. Naturalist, 1888. — The same. New Yoik, 1907. — 

 Owen, R., Odontography. London, lSiO-45.— Schlosser, M., Biol. Ceutralbl., 1890.— /Sfco«, IT'. B., 

 The evolution of the premolar teeth in the Mammalia. Proc. Acad. Nat. Sci. Philad., 1892. — 

 Wortman, J. L., The comparative anatomy of the teeth of the Vertebrata. Washington, 1886. 

 For a critique of the Cope-Osborn theory of the evolution of the molars, see Gregory, W. A'., in 

 Bull. Amer. Mus. Nat. Hist., 1910, pp. 184-194, and ibid., 1916. Also Gidley, J. II'., Evidence 

 bearing on Tooth-Cusp Development. Proc. Washington Acad. Sci., 1906, vol. viii. pp. 91-210. 



