60 Tlie Amei'ican Geologist. juiy, i8!» 
longitudinal striation, of which on the Sandusky form not a trace 
can be perceived. 
Comparing microscopical sections of the two forms, their struc- 
tural diversity is still more clearly exhibited. 
Transverse sections of the Sandusky specimens and of similar 
ones from the Helderberg group at Long lake near Alpena, 
present thin walled polygonal orifices \ of one millimeter in di- 
ameter and somewhat larger ones crowded around the monticules. 
In the angles between the tubes a small number of triangular or 
quadrangular cell spaces may be noticed, which must be taken 
for the young ordinary tubes. Besides these, also acanthopores 
are observable in distantly scattered position. In every one of 
the successive layers the tubes are near their upper termination 
subject to a slight incrassation, which thickened portion exhibits 
under the microscope its diaphanous sclerenchymal mass dotted 
by numerous dark punctations disposed in a double row around 
the circumference of the orifices. If sections are not thin enough 
for good transparency, these densely crowded punctations could 
sometimes be mistaken for cross bars extending from one side of 
a wall to the other, or for the channels of connecting pores, but 
in sections, sufficiently thin, it becomes plain above all doubt, 
that these dark dots are isolated punctiform, and never trav- 
erse the entire thickness of the walls. This punctation of the 
substance of the tubewalls, noticeable in cross sections, is also in 
longitudinal sections dimly perceptible, but only the mcrassated 
wall portions show this dotted structure; the intersected thin- 
walled parts of the tubes do not. 
In all the examined specimens were found developed trans- 
verse diaphragms, unequally distributed and rather remote in po- 
sition. 
Summing up the points of the preceding description, this form 
(Hall's Chatrtcs tennis) corresponds in all particulars with 
Nicholson's proposed subdivision of MonticnUpora, termed Mono- 
trypa excepting in the ])resence of a few acanthopores, said to be 
missing in Monotrypa. But this arbitrary restriction of generic 
limits, which is not fully proved even with regard to the type 
form Monotrypn undidata, of which I have specimens exhibiting 
distantly' scattered acanthopores, does not detain me from asso- 
ciating the described form with its nearest family relations and to 
introduce it under the name Monotrypa tennis Hall sp. , since the 
