METAZOA VERMES. 265 



VERMES. 



Section n and 12 (in part). 

 BRACHIOPODA. ATREMATA. 



The class of Brachiopoda, as already stated, is one of 

 the best for illustrating a classification based on the -stages 

 of growth and decline. 



The ancestral form, Paterina, which Beecher described 

 and figured, 1 is now considered by Walcott 2 to be identi- 

 cal with the genus Iphidea ; and in the Iphidea, Walcott 

 has found a rudimentary cardinal or hinge area. This 

 fact prevents the Iphidea and the Paterina, if they be the 

 same, from representing the most primitive form conceiva- 

 ble; that is, a shell without even the rudiment of a cardi- 

 nal area. There is little doubt, however, that such a 

 primitive form existed and in time will be discovered. 

 To such a genus, when found, the name of Paterina 

 should surely be given in consideration of Beecher's 

 classic researches on the class of Brachiopoda. Until this 

 new, theoretical form is brought to light, we will bear in 

 mind that the Iphidea is the simplest form yet discovered 

 and described, while at the same time we shall accept the 

 shell as described by Beecher under the name of Paterina 

 as representing the still more primitive form that is to be 

 discovered. 



This Paterina shell is simple in youth (PI. 647, fig. i, 

 P. labradorica Billings, pedicle valve) and also in maturity 

 (fig. 2, brachial valve), having two nearly equal valves 

 which are semi-elliptical in shape. The hinge line is 

 straight and nearly equal in length to the width of the 



1 Amer. Journ. Sci., (3), XLI, 1891 ; (3), XLIV, 1892. 



2 Proc. U. S. Nat. Mus., XIX, 1897, pp. 707-718. 



