BRACHIOPODA. 



135 



median ridge dividing the muscular impressions and supporting the cardinal 

 process. 



The structure of the brachial apparatus has been carefully re-examined, and 

 it is found that the crura are slender and rather long, slightly converging toward 

 their apices, forming an acute angle where they meet the primary lamellae; the 

 latter turn outward and backward, remaining widely separated throughout their 

 extent. The coil is lax, the ribbon making but about three volutions, and the 

 cones, though very slightly elevated, have their apices directed outward, toward 

 the lateral slopes of the pedicle- valve. These shells vary considerably in convex- 

 ity both naturally and from accidental compression, and where the internal 

 cavity is shallow the spirals may appear to be coiled almost in oblique planes. 



The umbonal curves of the primary lamellae are very broad and stout; the 

 loop arises on their posterior limb, broad and strong, its lateral proces.ses curv- 

 ing gently forward and thence upward, not as in Zygospira, nor as represented 

 in the original figure of these organs in Calospira concava, but elevated and 

 acutely angulated at the apex. 



Beyond the junction of the lateral processes the loop is continued as a simple 

 stem which is inclined backward and may have been extended to the surface of 

 the internal ridge on the pedicle-valve, as in Terebratula lepida. 



FIG. 123. FIG. 123. 



Fig. 122. The brachidiiim of Ccelospira concava, Il.ill. 



Fig. 123. Profile, showing the elevation of the loop. The stem of the loop is probably broken and therefore shorter 

 than is natural. ic.) 



In front of the base of the loop the primary lamelliB become at once narrow 

 and delicate, and it not infrequently happens, in preparations of the interior. 



