ORDOVICIAN FAUNAS. 139 
The specimens of this species which have been available for study 
are all imperfectly preserved, being present upon weathered surfaces 
of Trenton lmestone slabs. 
HYDROZOA. 
ACTINOSTROMA TRENTONENSIS nh. sp. 
Plate VI., Fig. 8, and Plate VII., Figs. 3-4. 
Description—Ceenosteum subglobose or pyriform, often exceedingly 
irregular in form. In cross-section, as exposed on weathered surfaces, 
it is seen to consist of a series of concentric layers or lamine, varying 
from 1 to 3 mm. in thickness, which, by the aid of a lens, are seen 
to be composed of minute, crowded, parallel fibres, whose direction 
is vertical to the surfaces of the lamine. In the thin, vertical section, 
under the microscope, the fibrous structure is conspicuously exhibited, 
the vertical fibres being irregularly joined together by vertical, lateral, 
plate-like expansions. In the thin, transverse sections the vertical 
fibres, with their lateral expansions, are seen to coalesce in a most 
irregular manner, leaving numerous, vertical openings, which are ex- 
eeedingly variable in form and size. 
The usual diameter of masses of this organism is from 25 to 40 mm. 
Remarks.—In the lower portion of the Trenton limestone, in strata 
which are probably of Black River age, certain beds are frequently 
more or less completely composed of the colonies of this organism, 
though upon the unweathered rock surfaces they often can scarcely 
be detected. On the weathered surfaces, however, they may usually 
be easily recognized by their texture and slightly different color, 
which distinguishes them from the matrix in which they are buried. 
The organism is clearly one of the Stromatoporoids, and has been 
placed provisionally in the genus Actinostroma, because of its con- 
spicous fibrous structure. It differs from the usual members of 
that genus, however, in the apparent absence, or at least small num- 
ber, of horizontal rods connecting the vertical fibres, which usually 
gives to the thin, vertical section of Actinostroma a reticulate ap- 
pearance. 
