316 



HISTORICAL GEOLOGY. 



and characteristic form of coral-making Bryozoan, called 

 from its perfect screw-like form, Archimedes (Fig. 241). 

 This abundant and easily recognized form is 

 wholly characteristic of the sub-Carboniferous. 

 By studying the diagram (Fig. 243) the main 

 facts regarding Echinoderms may be easily re- 

 membered. As before (page 279), the lower 

 shaded part represents stemmed, and the up- 

 per unshaded, the free forms. The Cystids, 

 it is seen, are confined to the Silurian, the 

 Blastids commence in the Silurian, continue 

 through the Devonian and Carboniferous, and 

 perish ; while the Crinids continue until now. 

 The Asteroids commence in the Lower Silu- 

 rian, the Echinoids in the Carboniferous, 

 and both continue until now the species, of 

 course, changing. As Blastids are very abun- 

 dant in the sub-Carboniferous, we give a fig- 

 ure (Fig. 242). 



Concerning Mollusca, we touch two points : 

 1. Fresh-water and land shells, which were in- 

 troduced in the Devonian, are more abundant 

 (Figs. 244, 245). 2. The Goniatites, first in- 

 troduced in the Devonian, are also more 

 numerous here (Fig. 246). 

 Concerning Crustacea, also two points : 

 1. While Trilobites continue under new 

 forms, ready to perish at the end of this 

 period, Lmiuloids, or horseshoe crabs, a 

 higher type, are here introduced (Fig. 247). 

 The transition from Trilobites to Limuloids 

 may be quite perfectly traced. 2. True typi- 

 cal crustaceans of the long-tailed kind (Ma- 

 crourans), such as shrimps and the like, were 

 first introduced here (Fig. 248). 



Insects, which first appeared in the Devo- Hail.) 



FIG. 241. 

 Archimedes 

 laxa. (After 

 Hall.) 



