170 



BRACHIOPODA 



which have been examined. This embryonic shell has been 

 termed the protegulum, and may sometimes be found at 

 the umbones of adult shells, but generally, owing to its 

 delicate nature, it has been worn off; it is semicircular or 

 semi-elliptical in form, with concentric lines of growth, and 

 is without an area ; it is composed of horny material, and 

 varies in size from '05 to "60 millimetre. From the con- 

 stancy of the occurrence of the protegulum it has been 

 inferred that the ancestral form of the Brachiopoda 

 possessed throughout life a shell similar to the protegulum; 



B 



Fig. 75. Iphidea [ = Pater ina] labradorica, from the Lower Cambrian 

 (Olenellus Beds). A, ventral valve. B, dorsal valve. Enlarged. 



but, at present, no brachiopod agreeing entirely with the 

 protegulum has been found ; for although Iphidea (fig. 75), 

 from the Lower Cambrian, is in many respects similar, yet 

 the possession of an area distinguishes it from a prote- 

 gulum. 



The Brachiopoda have been divided by nearly all 

 authors into two Classes, (1) the Inarticulata, (2) the 

 Articulata 1 , each of which may be divided into two 

 Orders. 



1 These classes have received other names ; the Inarticulata being 

 known by some authors as the Lyopomata, the Ecardines, the Pleuro- 

 pygla, or the Tretenterata ; and the Articulata as the Arthropomata, the 

 Testicardines, the Apygia, or the Clistenterata. 



