528 



GEOLOGY 



The Brachipods. The lower, inarticulate forms of brachiopods 

 which predominated in the Cambrian, continued through the Ordo- 



Fig. 390. Ordovician Brachiopods: a, Rafinesquina alternate (Emmons); 

 6, Platystrophia lynx (Eich.); c, Hebertella sinuata (Hall); d, H. sinuata 

 (Hall), interior view of the brachial valve, showing muscular impres- 

 sions and hinge; e, Schizotreta ovalis H. and C., a pedicle valve; /, Tre- 

 matis millepunctata Hall, pedicle view of a complete shell, showing the 

 unmodified notch-like pedicle opening; g, Lingula reclilateralis Emmons; 

 h-i, Strophomena subtenta Conrad, posterior view of a complete shell, 

 showing the hinge-line, etc., and the exterior of the concave pedicle 

 valve (i); /, Crania Icelia Hall, brachial views of four individuals at- 

 tached to another shell; k, Schizocrania filosa (Hall), a brachial valve; 

 I, Leptcena rhomboidalis Wilck, the pedicle valve; m, Orthis triccnarin 

 Conrad, exterior of the brachial valve and the cardinal area of the 

 pedicle valve; m, Orthis tricenaria Conrad, exterior of the brad i in I 

 valve and the cardinal area of the pedicle valve; n, Rhynchotrema 



Conrad; o, Dalmanella testudinaria (Dal.), brachial view; p, Plectanihon- 

 ites sericeus (Sow.), brachial view; q, Catazyga headi (Bill.), brachial 

 view; r, Zygospira recurvirostris (Hall), interior of a brachial valve, 

 showing the spiral brachidium in position. 



vician (and to the present time), but the higher, articulate forms 

 greatly outnumbered them. The expansion of the articulate types 

 was attended by a progressive evolution of the mode of articulation. 



