768 Report oF THE State GEOLoGIsT. 
supporting short, straight crura. The spiral ribbon makes not 
more than six volutions, forming very loose coils. There is no 
median septum. 
Type, Hyattella congesta, Conrad (sp.). 
Distribution. Middle Silurian. 
Dayia, Davidson. 1881. 
(Plate 32, figs. 21-25.) 
Shells small, subtrihedral in contour, with a very convex 
pedicle-valve which may be obscurely keeled along the middle 
and depressed laterally. Brachial valve convex posteriorly, but 
becoming concave over the anterior region, and bearing a well- 
developed median sinus. The hinge-line is short; the cardinal 
area absent. The umbo of the pedicle-valve is gibbous and its 
apex closely incurved, concealing the foramen. Deltidial plates 
were probably developed, but they appear to be invariably lost in 
separated valves. The delthyrium is wide, the teeth divergent, 
moderately conspicuous and unsupported by lamelle. In the 
bottom of the valve are two narrow, diverzent muscular grooves, 
bordered on their anterior edges by thickened ridges, both hav- 
ing the shape of a broad inverted V. 
In the brachial valve the character of the hinge-plate was 
probably simple, and supported by a median septum traversing 
about one-half the length of the valve; on either side of this 
septum are the lateral members of the adductor impression. The 
crura are short and straight; the primary lamellz are attached 
to them by a subrectangular curve and pass outward just within 
the margin of the valve. The spirals are but slightly elevated 
and have their apices directed outward toward the lateral slopes 
of the opposite valve. The ribbon makes but three or four turns, 
and its outer anterior edges are quite coarsely fimbriated. The 
jugum is situated anteriorly, taking its origin near the upward 
turn of the primary lamelle ; it is directed upward and backward, 
the lateral processes meeting at or just behind the center of the 
interior cavity. From the point of union proceeds a short, simple 
process, which does not make an angle with the rest of the 
jugum. 
Type, Dayia navicula, Sowerby (sp.). Wenlock limestone. 
But one species has been recognized. 
20 
