ORDOVICIAN FAUNAS. 161 
ZYGOSPIRA RECURVIROSTRA (Hall). 
Plate X., Figs. 23-26. 
1895. Zygosmra recurvirostra W. & 8., Pal. Minn., pt. I., p. 466, pl. 
34, figs. 38-41. 
Description.—Shell small, subcircular or longitudinally subovate in 
outline, subglobular; surface of both valves marked by twenty-four to 
twenty-eight rounded or subangular, radiating plications, which are 
crossed by fine, concentric lines of growth. Pedicle valve gibbous, with 
its greatest elevation near the centre; subcarinate near the beak, the 
keel becoming broader toward the front and forming a rather well- 
defined, more or less flat-topped median fold; beak small and pointed, 
incurved over the beak of the brachial valve. Brachial valve less con- 
~ vex than the other, marked by a rather broad, shallow, rounded me- 
.dian sinus, which corresponds with the fold of the pedicle valve and 
which reaches nearly to the beak. 
The dimensions of an average specimen are: length, 6 mm.; width, 
6 mm.; thickness, 4 mm. 
ZYGOSPIRA NICOLLETI (W. & S8.)- 
Plate X., Figs. 27-30. 
1892. Hallina mecolleti W. & S., Am. Geol., vol. IX., p. 293. 
' 1893. Hallina nicolleti W. & S., Pal. Minn., pt. L., p. 474, pl. 34, figs. 
59-62. 
1893. Zygospra nicolleti B. & S., Proc. Biol. Soc. Wash., vol. VIII., 
pp- 74, 81, pl. 10, fig. 23, pl. 11, figs. 11-12. 
Description.—Shell small, rostrate, elongate-oval to subcircular in 
outline, smooth. Pedicle valve subcarinate, with a slight mesial de- 
pression in the carina near the front margin; beak acute, not incurved, 
the lateral slopes, from the carina to the sides, concave. Brachial valve 
less convex than the pedicle, with a rounded, mesial sinus extending 
from near the beak to the front margin. 
11 
