BRACHIOPODA. 147 



length of the valve a median furrow begins on this ridge, widening ante- 

 riorly, and thus making a double keel over the forward parts of the shell. 

 The lateral slopes are broad and smooth, interrupted only in the umbo-lateral 

 regions by a short fold on each side, originating at the beak and lying 

 just within the margins. 



FlO. 133.^ FlO. 134. 



Front and proBle views of Cydotpira Usutcaia, Emmons. X 2. 



The brachial valve is slightly convex posteriorly, becoming concave medi- 

 ally over the pallial region. The median sinus bears a low fold corresponding 

 to the central groove of the opposite valve. On the interior of the pedicle- 

 valve the shell in the umbonal region is very thick, and in this thickened 

 portion the scar of the pedicle-muscle, and in front of it, the adductor scar 

 is excavated. At the anterior edge of the muscular area the shell becomes 

 suddenly and abruptly thinner, and thus that area lies on a well-developed, 

 solid platform. In the brachial valve the hinge-plate is small and sup- 

 ported by a low median septum which extends about two-thirds the length 

 of the valve. 



The crura diverge slightly as they pass downward, making a very low curve 

 or slight angulation at their union with the primary lamellaB. The spiral 

 ribbon is very delicate and quite short, making but two and one-half or three 

 volutions, which are almost circular. The interesting feature of these spirals is 

 that they are coiled in planes nearly parallel to the vertical axial plane of the 

 shell. The best of our preparations, which are transparencies, show quite 

 clearly that the apices of the spirals are very slightly introverted, and the 

 primary whorls are so close together that this slight introversion brings the 

 apices into approximation. This deviation from the vertical is apparently 

 quite normal, and is fully corroborated by the ZYGOSPiKA-like contour of the 

 shell ; and yet it would require but slight mechanical or casual disturbance of 



