THE LOWER FORMS OF LIFE. 53 



creatures comprised therein, existed in seas whose deposit, five miles 

 in thickness, is now at least nine miles more from the surface of 

 the earth. 



In the third geological period the Foraminifera are still found at 

 work. In the carboniferous limestone there are beds formed 

 entirely by a genus called Fusulina, which seems to have come into 

 being and gone out of it in this period of the earth's history, as it 

 is no more found in its crust, or in the waters of our time. 



As the name implies, these Fusulina are spindle-shaped, as shown 

 in Fig. 68, which is a cast of the chambers of the fossil. If cut 

 transversely across, a section will show its internal structure, as seen 



Fig. 68. Cast of the chambers of a Fusulina of simple type (after Ehrenberg) - a, median 

 segments 6, alar prolongation. 



Fig. 69. Transverse section of the centre of a Fusulina (after Carpenter), showing c c c, the 

 principal chambers a, the communication between the chambers -6 b, dark spots indi- 

 cating the origin of the alar prolongation (6, 68). 



in Fig. 69, which is very similar to that of Nummulites, a genus 

 existing in enormous numbers much more recently. Two other 

 genera of Foraminifera are also found associated with Fusulina in 

 these carboniferous limestones, viz., Eotalia and Textularia, both of 

 which have many living representatives in our own seas, and 

 examples of which were included in my last chapter (Figs. 55, 56, 

 60, 61, p. 42). 



At the end of the third geological period (see table) the form of 

 life appears to have undergone a great change, as we shall find more 

 particularly by and by. In the Lower Paleozoic, crustaceans reached 

 the highest amount of development, forty to every mile of rock. In 

 the Upper Paleozoic, fishes commence that remarkable numerical 

 development which reaches its highest point in the Jurassic 270 

 per mile. Reptiles, but little known in the Paleozoic, make a 

 wonderful advance in the Neozoic, and attain their highest range in 

 the Triassic over 90 per mile ; whilst mammals, commencing in 

 the Jurassic, where they are represented by seven species per mile 

 of rock, disappear entirely in the Cretaceous, and in the Tertiary are 

 found in the proportion of 34 to every mile of rock. There is thus 



