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 umbonal 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 face 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; they narrow with a slight twist just above their origin, as in 
the genera Rhynchospira and Trematospira, then broaden, curving ouHvard and 
* It will be observed that in the athyroid and retzioid genera, with the exception of Nucleospira, broad 
umbonal blades and a posterior position of the loop characterize the Carboniferous forms, while in the earlier 
faunas the species have narrow primary lainellfe and a medially situated loop. 
