ORDOVICIAN FAUNAS. 149 
valve. Surface of both valves marked by numerous, moderately-fine, 
radiating coste. Those of the pedicle valve usually imcreasing by 
intercalation, and usually arranged with one to six or eight smaller 
and shorter ones between each two larger and more prominent ones, 
the largest one of which often occupies the mesial line. On the 
brachial valve the radiating strie more frequently increase by divi- 
sion, and are usually more uniform in size. In addition to the radi- 
ating lines, the shell is marked by numerous, minute, regular, closely- 
arranged, concentric strize, which are visible only on well-preserved 
specimens by the aid of a magnifying glass. A few more or less dis- 
tinct, subimbricating lines of growth are also seen near the free mar- 
gins of adult shells. 
The approximate dimensions of a nearly-complete pedicle valve are : 
length 26 mm.; width, 31 mm. The species at times attains a much 
larger size than this, but the New Jersey specimens are usually smaller. 
Remarks.—This very characteristic Ordovician brachiopod occurs 
abundantly in some strata of the Trenton limestone of New Jersey. 
Usually, however, it is not an abundant form, and the specimens 
are imperfectly preserved. The shell is a variable one in most of its 
characters, but it may always be recognized by its convex pedicle 
valve, with conspicuously alternating, radiate markings. 
PLECTAMBONITES SERICEUS (Sowerby). 
Plate IX., Figs. 14-15. 
1873. Leptena sericea Meek, Pal. Ohio, vol. I., p. 70, pl. 5, figs. 
3.a-h. 
Description.—Shell subsemi-elliptical or subsemi-circular in out- 
line, coneavo-convex, hinge-line equaling or usually a little longer 
than the breadth of the valves in front; cardinal extremities varying 
from nearly rectangular to acutely angular, anterior and lateral mar- 
gins, forming together a nearly-regular, semi-circular curve. Pedicle 
valve moderately convex, nearly evenly, but gently, arched along the 
median line from the beak to the front; beak yery small, scarcely, 
f at all, distinct from the cardinal margin; cardinal area two to 
three times as high as that of the opposite valve, inclined backward 
or more or less nearly parallel to the plane of the valve; delthyrium 
