Relative Longevity of Species in the Mammalia and Testacea.—I called attention in 1830[5] to the fact, which had not at that time attracted notice, that the association in the Post-pliocene deposits of shells, exclusively of living species, with many extinct quadrupeds betokened a longevity of species in the testacea far exceeding that in the mammalia. Subsequent researches seem to show that this greater duration of the same specific forms in the class mollusca is dependent on a still more general law, namely, that the lower the grade of animals, or the greater the simplicity of their structure, the more persistent are they in general in their specific characters throughout vast periods of time. Not only have the invertebrata, as shown by geological data, altered at a less rapid rate than the vertebrata, but if we take one of the classes of the former, as for example the mollusca, we find those of more simple structure to have varied at a slower rate than those of a higher and more complex organisation; the Brachiopoda, for example, more slowly than the lamellibranchiate bivalves, while the latter have been more persistent than the univalves, whether gasteropoda or cephalopoda. In like manner the specific identity of the characters of the foraminifera, which are among the lowest types of the invertebrata, has outlasted that of the mollusca in an equally decided manner. Teeth of Post-pliocene Mammalia.—To those who have never studied comparative anatomy, it may seem scarcely credible that a single bone taken from any part of the skeleton may enable a skilful osteologist to distinguish, in many cases, the genus, and sometimes the species, of quadrupeds to which it belonged. Although few geologists can aspire to such knowledge, which must be the result of long practice and study, they will nevertheless derive great advantage from learning, what is comparatively an easy task, to distinguish the principal divisions of the mammalia by the forms and characters of their teeth. Fig. 93: Elephas primigenius (or Mammoth) molar of upper jaw, right side. Post-pliocene; Fig. 94: Elephas antiquus, Falconer. Penultimate molar. Post-pliocene and Pliocene. Figures 93 through 105 represent the teeth of some of the more common species and genera found in alluvial and cavern deposits. Figs. 95 to 100: Teeth of extinct mammalia. Figs. 101 to 105: Teeth of extinct mammalia. On comparing the grinding surfaces of the corresponding molars of the three species of elephants, Figs. 93, 94, 95 it will be seen that the folds of enamel are most numerous in the mammoth, fewer and wider, or more open, in _E. antiquus_; and most open and fewest in _E. meridionalis._ It will be also seen that the enamel in the molar of the _Rhinoceros tichorhinus_ (Fig. 97), is much thicker than in that of the _Rhinoceros leptorhinus_ (Fig. 96). [1] Sir John Lubbock, Pre-historic Times, p. 3, 1865.