l82 AN INTRODUCTION TO THE STUDY OF FOSSILS 



(brachia) of the lophophore ; because this valve supports the 

 brachia it is often called the brachial valve, 



Externallv the shell is marked by concentric lines (growth 



Fig 73 a — Tinhratuhna septentrionalis Couthoy, from the coast of Maine. A, in- 

 side of the brachial \ alve. B, inside of pedicle valve. C, side view of the animal 

 fastened to a rock by the fleshy pedicle. D, soft body of animal in brachial valve 

 with pedicle valve removed. E, an enlarged section of the brachium along line 

 a-b in figure D, showing the ciliated tentacles {te.) which urge the food into the cili- 

 ated food grooves ij.g) ; a-b, line of section, Fig. E ; a.m., adductor muscle 

 scar; ft., brachial valve ; ftr., brachidium ; ftra., the horseshoe-shaped lophophore 

 with its branches or arms, — the brachia ; c.p., cardinal process, place for the attach- 

 ment of the diductor muscles ; d.m., diductor muscle scar ; f.g., food groove ; 

 Ip., lip; mo., mouth; ^., pedicle valve ; /»e., pedicle; /)c.o., pedicle opening for 

 passage of pedicle; s., socket into which fits the tooth {t.) ; 5/., sinus; /., tooth; 

 te., tentacles. All natural size, e.xcept E. 



lines) which represent the successive stages of growth ; as the 

 shell grew new layers were added to the inside, projecting be- 

 yond the preceding layers and forming thus a series of shell ex- 

 tensions. (See also pp. 215, 216.) Since the pedicle is present 

 in the youngest shell-secreting stage of the brachiopod, and the 

 animal when young must have had a minute pedicle, growth must 



