BRAcHIOPODA. 809 
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 supported 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 lamellz. The spiral ribbon is very delicate and quite 
short, making but two and one-half or three volutions, which are 
almost circular. These spirals are coiled in planes nearly 
parallel to the vertical axial plane of the shell, being very 
slightly introverted, and the primary whorls so close together 
that this slight introversion brings the apices into approximation. 
There is as yet no satisfactory proof of the existence of a jugum; 
indeed, the evidence derived from a number of transparent pre- 
parations is decidedly negative upon this point. Where the 
crura are attached to the primary lamell, the ribbon is broad- 
ened, and just in front of these points there appear to have been 
two short convergent apophyses which may be construed as dis- 
crete elements of a jugum, but the shell appears to have been 
actually ajugate. 
Subgenus Protozyga, Hall. 1893. 
(Plate 40, figs. 4, 5.) 
Shells small, subplano-convex, the pedicle-valve being the 
deeper. The ante-lateral margins of the valve bear evidence of 
broad, coarse plications. 
The brachial valve has a simply divided hinge-plate, upon these 
divisions resting the two short convergent crura; joining the lat- 
ter at alow angle, the primary lamellz diverge laterally, con- 
verge slightly toward their anterior margins, thence curve verti- 
cally upward, nearly touching the inner surface of the pedicle- 
valve and very gradually approaching each other. The ribbon is 
continued with a decided internal inclination, until it completes 
102 61 
