3 82 



ORDOVICIAN PERIOD 



they are rather rare in the limestone beds of the early and middle 

 Ordovician, and more abundant in the later shales. 



Brachiopods were still very abundant. Some were very similar 

 to those of the Cambrian; but the higher, articulate forms (valves 

 of the shell articulating) greatly outnumbered them. Among the 

 articulate forms, the length of the hinge was increased, apparently 

 affording a better means of resisting the attempts of enemies to 

 reach them by sliding or rotating the valves past one another 

 (i and p, Fig. 340), while in others the margins of the valves were 



Fig. 340. ORDOVICIAN BRACHIOPODS: a, Rafinesqnina alternata (Emmons); b, 

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

 view of the brachial valve, showing muscular impressions and hinge; e, Schizotreta 

 avails H. and C., a pedicle valve;/, Trematis millepunctata Hall, pedicle view of a 

 complete shell, showing the unmodified notch-like pedicle opening; g, Lingula 

 rectilateralis Emmons; h-i, Strophomena subtenta Conrad, posterior view of a com- 

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

 (i) ;j, Crania Icelia Hall, brachial views of four individuals attached to another shell ; 

 k, Schizocrania filosa (Hall), a brachial valve; /, Leptcena rhomboidalis Wilck, the 

 pedicle valve; m, Orthis tricenaria Conrad, exterior of the brachial valve and the 

 cardinal area of the pedicle valve; n, Rhynchotrema capax Conrad; o, Dalmanella 

 testudinaria (Dal.), brachial view; p, Plcctambonites sericeus (Sow.), brachial view; 

 q, Catazyga headi (Bill.), brachial view; r, Zygospira recurviroslris (Hall), interior of 

 a brachial valve, showing the spiral brachidium in position. Compare Fig. 324. 



