i82 AN INTRODUCTION TO THE STUDY OF FOSSILS 



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

 brachia it is often called the brachial valve. 



Externally the shell is marked by concentric lines (growth 



Fig. 73 a. — Terebratidina se ptentrionalis Couthoy, from the coast of Maine. ^, in- 

 side of the brachial valve. 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 {f.g.) ; a-b, line of section, Fig. E; a.m., adductor muscle 

 scar; ft., brachial valve ; />r., brachidium ; tra., the horseshoe-shaped lophophore 

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

 ment of the diductor muscles ; J.W., diductor muscle scar; J.g., food groove; 

 Ip., lip; mo., mouth; />., pedicle valve; ^e., pedicle; ^e.o., pedicle opening for 

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

 te., tentacles. All natural size, except 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, grow^th must 



