298 HISTORICAL GEOLOGY 



(6) LAMP SHELLS (Brachiopods). 



These animals are now rare and are not well known 

 even by their English name. They are inclosed in a 

 pair of shells (Figs. 369 and 396), and hence are easily 

 confused with the lowest group 

 of mollusks; they may be dis- 

 tinguished from 

 the latter by the 

 bilateral symme- 

 try of their shells. 

 A pair of spiral 



FIG. 308. Internal structure of the shells armlike Organs 

 of brachiopods. . , 



aids in getting 



food to the mouth. In the earlier forms these spirals 

 were soft and hence were not fossilized. Later, some 

 of them came to have solid supports of lime carbonate 

 which were durable (Fig. 308). Fossil brachiopods 

 are varied in shape and are abundant in many of the 

 older rocks. 



(7) MOLLUSKS. 



In this group are found not only well-developed internal 

 organs, but, in the higher types, even a distinct head, 

 eyes, and teeth. All are soft-bodied, but they are 

 usually protected by a hard, limy shell. 



(a) Bivalves (Pelecypods). 



Forms like the common oyster and clam, provided 

 with two shells, usually nearly alike, which hinge 

 together on one side (Figs. 352 and 374). 



(6) Snail group (Gastropods). 



The snail and its relatives have a single conical 

 shell which is almost always twisted or coiled into 

 a spiral form. The earliest and most primitive 

 types are merely cap-shaped (Fig. 336), but later 

 species developed a great variety of coiled shells 



