METAZOA VERMES. 277 



Rhynchonella peregrina (No. 676) from the Cretaceous, 

 Rhynchonella octoplicata (No. 677), and Rhynchonella 

 psittacea Dvds. ( = ffemithyris psittacea Chemnitz) (No. 

 678 ; No. 679, shells of the same), living to-day, show 

 no marked difference in form from the primitive Pro- 

 torhyncha of the Silurian formation, although the size 

 varies, Rhynchonella peregrina being the giant of the 

 genus. This species exhibits the curved hinge line (No. 

 676), the prominent beak, and the pedicle opening which 

 is partly closed by two plates called by Hall and Clarke 

 deltaria. The origin of these plates is entirely different 

 from that of the single plate, the deltidium. As we have 

 already seen in the Protremata (see p. 272), the deltidium 

 is a primitive character arising in the embryo or early 

 nepionic stage of development, while the deltaria are 

 formed in the neanic or the ephebic stage and are made 

 by extensions of the mantle. Owing to this difference in 

 origin and structure, and in order to save confusion, we 

 prefer the name of deltaria to deltidial plates given by 

 many naturalists. 



The smooth, unornamented condition of the young 

 Rhynchonella shell is seen in No. 677, while the later 

 stages are marked by ribs and flutings. Along the mar- 

 gin, the layers are crowded closely together, an indication 

 of the gerontic stage. 



The specimen of Rhynchonella psittacea Dvds. (No. 

 678) is attached to a pebble. Unlike Protorhyncha, this 

 genus has the arms supported by two short, simply 

 curved processes, the crura (see No. 679, f; PI. 680), 

 which are attached to the brachial valve (see No. 678 

 specimen with arms extended). 



Specialization of parts is finely illustrated in this order 

 by the structure of the brachia and their internal limy 

 supports or brachidia. Beginning with the simple, curved 

 crura of the Rhynchonellae (PI. 680, crura with the fleshy 

 arms coiled), we pass to Centronella (C. glansfagea Bill- 

 ings; PL 68 1, fig. i, dorsal view; fig. 2, ventral view; 



