280 



HISTORICAL GEOLOGY. 



valves are upper and lower, or a back-piece and a breast- 

 plate. The deeper and more projecting valve is the ven- 

 tral. From the point of this valve comes out a fleshy 

 cord, by which it is attached. It is this which gives it 

 the name of lamp-shells, on account 

 of its resemblance to the ancient lamp 

 (Fig. 187). A large portion of the in- 

 terior of the shell is occupied by long, 

 spiral, fringed arms. It is these which 

 give the name of brachiopod (arm-feet), 

 although they are really gills. These are attached to 

 complex, and sometimes spiral, bony pieces. Fig. 188 is 

 a living brachiopod, showing structure. These shells 



FIG.' 187. Living bra- 

 chiopod. Side view. 



FIG. 188. A living brachiopod Terebratnla flavescens. 



are so extremely abundant in Paleozoic, especially Silu- 

 rian rocks, that these rocks may often be identified by 

 them. In Figs. 189, 190, we give 

 two of the most common forms. 

 Are there any characters by whicli 

 Silurian brachiopods can be easily 

 distinguished ? Not by the un- 

 trained eye. Yet the square shoul- 

 dered forms, like those figured here, 

 are very characteristic of Paleozoic, though not of Silu- 

 rian. 



Liamellibraiiclis and Gasteropods. The ordinary 

 bivalve-shells (Lamellibranchs], and the univalves or gas- 



Fio. 189.- -Silurian brachio- 

 pods ; OrthisDavidsonii. 



