172 



NORTH AMERICAN INDEX FOSSILS. 



lines are more or less conspicuous. Besides this ornamentation, 

 shells frequently bear folds or plications radiating from the region of 

 the beak to the anterior portion of the valves {Spirifer, Fig. 218, 

 Magellania, Fig. 219, 4-5). When these are fine they are gener- 

 ally spoken of as striations. Frequently the striae alternate in size. 

 They may increase by intercalation or implantation of new ones 

 between the diverging older ones, or these latter may bifurcate. 

 (See Figs. 251 to 275 for variations). Frequently the median line 

 of the brachial valve bears a much 



A 



larger fold called the median fold, 

 while the corresponding large depres- 

 sion on the pedicle valve is called the 

 median sinus {Spirifer, Fig. 409). In 

 some forms the sinus and fold are re- 

 versed in position {Anastrophia, Gypi- 

 dula, Enteletes, etc.), while in others 

 they are entirely wanting (Whitfield- 

 ella). 



■ The length or height of the shell is 

 the distance from the hinge line, or FlG - 2I 9- 1, Delthyrium of 

 line of junction of the valves, through y°™sX*J>»<*<>»f" without delti- 



.,..,. , 1, , dial P la t e s; 2, the same at a later 



the middle of the shell to the opposite stage with two tria n g uiar deltidial 



edge Or front (A-B in Fig. 2l8). The plates ; 3, the same after completed 

 width is in the direction at right an- growth ; showing joining of deltidial 

 gles to this or on a line extending plates ' and limi * a l tio " ° f P edicle 



' . opening to ventral beak (foramen); 



from side to side of the shell [C-D in 4> dorsal view of Mage i lania fiaves . 



Fig. 2l8). The thickness is in the cens, showing completed deltidial 



direction perpendicular to the plane P lates dd \ 5> P rofile of same ; w, 

 of length and width through the cen- ventral ^ pedicle ^ valve; ds > dorsal 

 ter of the valves. 



There is usually a flattened area at 

 the posterior portion of each valve between the beak (a) and the 

 hinge line (h) and extending to the cardmal angles ; this is the 

 cardinal area [ca in Fig. 218). The cardinal area of the pedicle 

 valve is the higher. 



In the Articulata a pair of teeth on the posterior portion of the 

 pedicle valve fit into corresponding sockets on the brachial valve. 

 Fitting between the teeth of the pedicle valve is a short prolonga- 

 tion of the brachial valve, the cardmal process (Fig. 26 1). Teeth and 

 sockets are usually absent in the Inarticulata. The dental sockets are 



(brachial) valve ; /, pedicle, 

 ter Beecher). 



(Af- 



