DISCINID A. 69 
Discina crassa, ffall (sp.). Pl. VI, figs. 6, 7. 
Orpicuna ? crassa, Hall. Pal. New York, vol. i, p. 290, pl. Ixxix, fig. 8 a, b, 1847. 
— — Murchison and Salter. Quart. Journal Geol. Soc., vol. vii, p. 151, 
fig. 7, and pl. x, figs. 3, 4, 1851. 
DISCINA — Salter. Siluria, 2nd ed., p. 542, 1859. 
Spec. Char. Ovate-orbicular or almost round, rather narrower posteriorly ; upper or 
free valve slightly convex and depressed, the mcurved apex close to the posterior margin ; 
surface marked with numerous very fine thread-like radiating strie, which increase in 
number by numerous interpolations near the margin, the whole being crossed by very fine 
concentric lines and wrinkles or rugze ; lower valve not known. Two specimens measured— 
Length 6, width 53 lines. 
BF EMG ROS TE 
Ods. Prof. Hall describes his species as follows—“ Ovate-orbicular, with the apex 
near the narrower extremity ; apex obtuse ; surface marked by strong concentric wrinkles 
and fine radiating striz.” And Mr. Salter observes, in his description of the Ayrshire 
specimens, that they appear to be exactly the same with some others from the Hudson 
River group, two miles N.E. of Troy, New York. 
Position and Locality. In England it occurs in the Llandeilo flags of Builth (Geological 
Survey); in the Caradoc beds near Llanfyllin; also in the same rock at Gretton, 
Shropshire. In Scotland it is found in the Caradoc beds at Ardwell and Penwhapple 
Glen, Ayrshire. 
Discina (‘TREMaAtIS) PuNcTATA, Sow. (sp.) Pl. VI, fig. 9. 
OxzBicuLa ? punctata, Sow. (O. granulata in the description, O. punctata in the ex- 
planation of plate.) Silurian System, p. 636, pl. xx, fig. 5, 1839. 
TREMATIS _ Sharpe. Quarterly Journal of Geological Society, vol. v, p. 69, 
1847, 
Discrna, _ Salter. Siluria, 2nd ed., pl. v, fig. 17, 1859. 
Spec. Char. Nearly circular in outline, or slightly ovate; upper or free valve 
moderately convex, the rounded apex being rather close to the posterior margin. External 
surface covered with raised, delicate, thread-like ridges, forming all over the shell a net- 
work of minute six-sided cells, the bottom of the cells being flat and margined by 
the slightly raised hexagonal ridge. ‘The lower or foraminated valve is conoidal or 
slightly convex, with a sunk oval-shaped slit, commencing at about the. middle of the 
