BRACHIOPODA. 107 



In the brachial valve the hinge-plate is subquadrate on its upper surface, its 

 posterior margin somewhat crescentic, the horns of the crescent extending into 

 the ambonal cavity of the opposite valve ; this character, however, is not so 

 highly developed as in Eumetria. The structure of this plate appears to be 

 essentially similar to that of Hustedia ; at all events, the tent-shaped crural 

 supports of Eumetria are absent ; there is, however, no trace here of the lig- 

 ulate, curved process which occurs in Hustedia, but the median portion of the 

 upper faco is convex and the lateral portions deeply grooved and bounded on 

 the outside by the elevated crural bases. The hinge-plate is supported by a 

 strong median septum which extends for nearly two-thirds the length of the 

 valve. It is most highly elevated near the middle of its length, where it ex- 

 tends vertically about one-fifth of the distance across the internal cavity; thence 

 it tapers rapidly to its anterior extremity. 



The brachidium has been reconstructed from serial transverse sections 

 of the shell in several directions, and the following description may be 

 relied upon as approximately accurate. The umbonal blades of the primary 

 lamellae are comparatively narrow and considerably incurved at their apices, 

 where attached to the long crura, as in Eumetria. The loop is situated 

 well forward, just behind the center of the lamellae ;* its lateral branches are 

 erect and long ; tliey narrow with a slight twist just above their origin, as in 

 the genera Rhynohospira and Trematospira, then broaden, curving outward and 



no. 99. FIG- lOO- 



The loop of Retzia Adrieni, de Vernenll, as reconstructed from consecutive sections, (C.) 



• It will be observed that in the athyroiii and i-etzioid g-enera, with the exception of Nuclkospiha, bioad 

 umbonal bliuies and a i)Osterioi- i)08ition of the loop charactenze the Carboniferous forms, while in the earlier 

 faunaH the species have nai-row primary lamellse and a medially situated loop. 



