406 



GEOLOGY. 



Silurian times they often became curiously constricted (Fig. 187, m-ri). 

 This appears to have been a protective device and implies that the 

 creature, although a member of the dominant type, had aggressive 

 foes. The device is noted chiefly in the rather small curved or coiled 

 forms, and may signify no more than the attacks of superior cephalo- 



c a e 



Fig. 188. — Corals: a, Favosites occidens Whit.; b, Syringopora verticillafa Goldf.; 

 c, Haly sites catenulatus linn.; d, Goniophyllum pyramidale (His.); e, Zaphrentis 

 umbonata Roming. Bryozoa: f, g, Fenestella pnrvulipora Hall. 



pods. It probably means increased slowness and inability to escape 

 by flight, for the animal could hardly have used his locomotive organs 

 to advantage through such constricted openings. The device did 

 not long continue, and it doubtless proved but a poor advantage, 

 and that for a brief season only when conditions were peculiar. 



The slow advance of the gastropods and pelecypods. — Although the 

 gastropods - were fairly well represented in the Cambrian period, and 

 quite amply in the Ordovician, they developed only a measured impor- 



