FOSSIL LAMP-SHELLS. 



229 



chambers ; so that this ancient genus of Brachiopods 

 takes its name from the circumstance. In the Rhyn- 

 c/ionellidcs there are two internal short, slender, and 

 curved plates. What we have to note, however, is 



Fig. 210. Rhyachonella (Car- 

 boniferous limestone). 



Fig. 211. Productus punctata (Car- 

 boniferous limestone). 



that all of these mechanical structures are merely 

 adaptations, to better enable the " arms " to do their 

 work effectively. It is in the smaller or dorsal valve 



Fig. 212. Productus giganteus (Carboniferous limestone). 



that this skeleton is usually seen. No skeleton is 

 found within the genera Lingula, Lingulella, Obolus, 

 Discina, Crania, etc. In the Terebratulce it is often 



