AGES OF NATURE. 



201 



the Jnoceramus (5), the Hippurites (c), and peculiar Spon- 

 dyli, with long spines (d). There is also a great variety of 

 Gasteropods, among which are some peculiar forms of Pleu- 



b c d e 



a 



Fig. 1 66. 



rotomaria (e). The Radiates are not inferior to the others 

 in variety.* 



489. TERTIARY AGE. Reign of Mammals. The most 

 significant characteristic of the Tertiary faunas is their 

 great resemblance to those of the present epoch. The ani- 

 mals belong in general to the same families, and mostly to the 

 same genera, differing only as to the species. And the spe- 

 cific differences are sometimes so slightly marked, that a 

 considerable familiarity with the subject is required, in order 

 readily to detect them. Many of the most abundant 

 types of former epochs have now disappeared. The 

 changes are especially striking among the Mollusks, the 

 two great families of Ammonites and Belemnites, which 

 present such an astonishing variety in the Oolitic and Creta- 

 ceous epochs, being now completely wanting. Changes of 

 no less importance take place among the Fishes, which are 

 for the most part covered with horny scales, like those of 

 the actual epoch, while in earlier ages they were generally 

 covered with enamel. Among the Radiata, we see the 

 family of Crinoids reduced to a very few species, while, on 

 the other hand, a great number of new Star-fishes and Sea- 

 urchins make their appearance. There are besides, innu- 

 merable remains of a very peculiar type of animals, almost 



* (a) Diploctenium cordalum ; (b) Marsupites ; (c) Salenia ; (d) Gale- 

 rites : (e) Micraster cor-anguinum. 



