i 
I 
I Silurian Fossils 105 
I dense, the corallites appearing as minute tubes traversing the dense 
substance at angles very oblique to the surface, gradually approach- 
i ing the latter. Near the surface the tubes widen rapidly laterally, 
attaining a width of about half a millimeter at the apertures; they 
I are very oblique to the surface. The apertures are strongly com- 
pressed vertically, resulting in an obliquely directed slit; the lower 
edge of the aperture gradually rises above the surface of the frond, 
the calyx widening from the tubular portion toward the aperture; 
near the aperture a cross-section of the lower edge is distinctly 
convex along the middle and slightly concave toward the sides 
: resulting in a more prominent elevation of the median part of the 
wall. The outline of the lower margin of the aperture is concave 
along the median part, often becoming convex at the sides where 
the sides of the lower edge of the aperture rest upon the general 
surface of the frond. In well preserved specimens the concavity 
of the outline of the lower margin of the aperture often is slight, 
but in worn specimens the median parts have suffered most, and 
the outline of this part of the aperture is then more strongly con- 
cave, or even deeply V-shaped. The upper wall of the aperture 
is formed by the general surface of the frond. No septal spines 
or pores or longitudinal ridges were noticed along the upper wall 
where the upper lower edge of the aperture had been weathered 
away. The internal structure between the tubular passages of the 
corallites is unknown; the cell walls apparently are thick walled 
here but the structure is not clearly defined in the specimens at 
hand. About 7 apertures in a width of 5 mm. 
Brownsport bed: near the home of E. Duncan, on Short creek, 
one and one-half miles east of Linden, Tennessee. 
Among the species referred to Pachypora, there is a group char- 
acterized by the sharp edge of the thin, strongly flattened, more or 
less appressed, lower lip, which may be indented with one or two 
emarginations, or may weather to a deeply indented V-shaped form. 
Septal spines are wanting. There is no indication of an internal 
process or septal ridge. This group includes, in addition to Pachy- 
pora platys, Aso Pachypora frondosa, Nicholson, and probably also 
Pachypora fisheri, Billings. For this group the term Platyaxum 
is here used. 
