AGES OF NATURE. 



233 



the Hippurites^ (c,) and peculiar Spondyli, with long 

 spines, (d.) There is also a great variety of Gastero- 

 pods, among which are some peculiar forms of Pleu- 



a 



Fig. 166. 



rfitomaria, (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 

 1o the same genera, differing only as to the species. And 

 ihe specific 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 Cretaceous epochs, 

 being now completely wanting. Changes of no less impor- 

 tance take place among the Fishes, which are for the most 

 part covered with horny scales, like those of the present 

 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, innumerable 



* (a) Diploctenium cor datum ; (b) Marsupites ; (c) Salenia ; (d) Ga 

 lerites ; (e) Micraster cor- anguinum. 



20* 



