426 



HISTORICAL GEOLOGY. 



Rhynchonellse (Fig. 411); are extended toward the front of the shell, and bent back and 

 united, forming a loop, in Terebratula, Magellania, etc. (Figs. 403, 404, and 402); or are 

 extended forward and coiled in variously shaped spiral coils, as in Spirifer, Atrypa, 

 etc. (Figs. 405, 408). In many extinct genera (Orthis, Strophomena, etc.) there are no 

 calcified arm-supports. These arms are covered with vibrating cilia, which serve to keep 

 up a current of water over or through the branchial cavity of the animal. 

 A few of the species are represented in Figs. 402-430 : — 



402-421. 



402 



Brachiopods. — Fig. 402, Magellania flavescens; 403, loop of Terebratula vitrea; 404, id. Terebratulina caput- 

 Berpentis; 405, Spirifer striatus; 406, same, interior of dorsal valve; 407, Athyris concentrica; 408,409, 

 Atrypa reticularis, the latter dorsal valve; 410, Rhynchonella psittacea, showing the spiral arms of the 

 animal; 411, id. dorsal valve; 412, id. ventral; 413, Strophomena planumbona; 414, id. dorsal valve; 41.5, 

 id. ventral; 416, Plectambonites transversalis ; 417, id. dorsal valve; 418, id. ventral; 419, Orthis stria- 

 tula; 420, id. dorsal valve; 421, id. ventral. 



