Report of the State SOLO 1ST. 457 



valve, because of the location of the principal nerve-ganglia on 



this side of the body. 



The. opposite usually smaller and imperforate valve, is known as 

 the brachial vatoe,a& to it areattacL mI the spiral arms oi brachia. 

 This is the dorsal, socket or hcemal vafox of some writers. 



Figs. 9.— Front and profile \ ews of Terebratula. 



These valves usually have an articulated hinge situated 

 beneath the beaks; sometimes they are held together in simple 

 apposition* by muscular contraction, but whenever, and to what- 

 ever extent the valves are opened by the animal, the basis of the 

 movement is at this posterior margin of the shell. 



In discussing the shells of the brachiopods a simple nomencla- 

 ture has been adopted. 



Most writers are agreed in orienting a shell with its beak- 

 upward. That portion of the valves thus lying above a horizon- 



Fig. 10.- Orthisina. FlG . 11.- Plectambcnitea. 



taJ axis (C— D) passing through the center of the valves, is the 

 posterior portion, that below this axis the anterior portion, The 

 shell is also divided by a vertical axis (A-B) into equal lateral 

 parts. The apex of the valves is their acute or obtuse point of 



19 



