440 MOLLUSCA. 



loop, of a very simple character. In these cases it is only 

 at their bases that the arms are supported, and they are 

 therefore more or less movable. In other cases, as in the 

 Spiriferidce (fig. 279), the arms must have been immovable, 

 as they are supported by two thin spirally-rolled lamellae, 

 which form two calcareous spires in the interior of the dor- 

 sal valve. In some cases, the whorls of these spires are in 

 turn furnished with minute calcareous spines, showing that 

 the cirri of the arms were also supported by an internal 



Fig. 279. Spirifera hysterica Carboniferous. The right-hand figure shows the interior of 

 the dorsal valve, with the calcareous spires for the support of the arms. 



skeleton. The form and development of the calcareous sup- 

 ports of the arms, though liable to vary with age, neverthe- 

 less furnish important characters in the discrimination of 

 fossil Brachiopods. 



The Brachiopoda may be divided into the two orders of 

 the Inarticulata (or Tretenterata) and the Articulata (or 

 Clistenterata). 



In the first of these orders (Inarticulata) the valves of the 

 shell are not united along the hinge-line, the mantle-lobes 

 are completely free, and the intestine terminates in a dis- 

 tinct anus. In this division are included the three families 

 of the Craniadce, Discinidce, and Lingulidce all very ancient, 

 and all represented at the present day by living forms to- 

 gether with the Silurian family of the Trimerellidce. 



In the second order (Articulata) the valves of the shell 

 are united by teeth along the hinge-line, the lobes of the 

 mantle are not completely free, and the intestine ends 

 blindly. In this division are included the living families 

 of the Terebratulidce, EJiynchonellidce, and the Thecidiidce, 

 and the extinct families of the Spiriferidce, Pentameridce, 

 Strophomenidce, and Productidce. In the first two of these 



