790 Report or THE StatTE Guoxoaist. 
Hustepra, 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 nighly elevated 
near the middle of itslength where it extends 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 lamellz are comparatively narrow 
and considerably incurved at their apices, where attached to the 
long crura, as in Eumetria. The jugum is situated well forward, 
Fia. 857. Fia. 358. 
The jugum of Retzia Adrieni, de Verneuil, as reconstructed from consecutive sections. 
just behind the center of the lamellz; its lateral branches are 
erect and long ; they narrow with a slight twist just above their 
origin, as in the genera Raynonosprra and Trematosprira, then 
broaden, curving outward and thence inward to their point of 
union The stem is short, making an angle with the lateral 
branches, and is directed backward. It reaches the level of the 
crura at a considerable distance in front of them and is there 
bifurcated, each arm making a slight double or sigmoid curve. 
These arms are, however, too short to reach the umbonal blades. 
The stem itself is continued for a short distarice above the point 
of bifurcation. The spiral ribbons make ten or eleven volutions 
in full-grown individuals. Fimbrizw are absent from both the 
spirals and jugum. 
Type, Retzia Adrieni, de Verneuil. Middle Devonian. 
But one species is known. 
42 
— 
