238 iM. Safford on the Genus Tetradium. 
we think referable to this genus. It differs from T. fibratum in 
the following particulars: the tubes are not as uniformly four- 
sided, nor are they arranged with equal regularity ; the walls are 
more strongly rugose; the lamellze appear to have been more 
delicate, and are generally not to be seen; traces of them how- 
ever can, in most instances, be found upon close examination. 
The four-sided character of the tubes is sufficiently well marked 
to justify this reference, in connection with the fact that traces of 
the lamelle can often be detected. 
This.species is associated with the last,and occurs, in addition, 
lower in the series, with Columnaria ulveolata Hall. It is a 
common fossil in our Central Basin. 
3. T. apertum Safford—Tubes isolated or fasciculated, or else 
united in linear series which often intersect, forming irregular 
reticulations ; breadth of tubes about half a line; lamellz as in 
T. fibratum. 
This species includes certain open, loosely constructed corals 
which belong to this genus. Two varieties may be designated. 
These appear to run into each other in some specimens, thoug 
it may be found necessary hereafter to separate them. 
a) Masses composed of separate tubes occasionally united by 
their sides. These forms often resemble Syringopora. 
) Masses composed of tubes arranged in linear series, the lat- 
ter intersecting and forming masses like those of Hulysites caten- 
wlatus Linn. 
Should it be found necessary to separate these varieties, the 
first ey be designated 7. laxum and the second 7. apertum. 
We have observed no characters, with the exception of the 
=“ mode of growth which separate this species from 7. fibra- 
Ne | 
The first variety is abundant in the middle part of the Lower 
Silurian series of Middle Tennessee. The second is found in the 
upper half as well as near the base. We have observed the 
same species in Kentucky. «it 
4. T. minus Safford—We include in this species massive speci 
mens, (generally small,) the tubes of which are only from 4th to 
4d of a line in breadth. The tubes in some specimens are quite 
regular, in others, though generally four-sided, are more or less 
irregular and have the aspect on the upper surface of Cheetetes. 
ee as in T. fibratum. af 
e have occasionally seen this species in the upper division: 
the Lower Silurian can in Middle Reset well as 2 
Kentucky. 
