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 lamellm are very broad and stout; the 
loop arises on their posterior limb, broad and strong, its lateral processes curv¬ 
ing gently forward and thence upward, not as in Zygospira, nor as represented 
in the original figure of these organs in Cizlospira 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. 122. 
Fig. 123. 
Fig. 122. The brachidium of Ccslospira concava, Hall. 
Fig. 123. Profile, showing the elevation of the loop. The stem of the loop is probably broken and therefore shorter 
than is natural. 
(C.) 
In front of the base of the loop the primary lamellae become at once narrow 
and delicate, and it not infrequently happens, in preparations of the interior. 
