772 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
they are alternated on each side of the median line and adjacent to each other on 
this line. The tubes are visible, flat, separated by a scarcely salient thread. The 
basal lamella is striated transversally ; the zoarial margins are thick and distinct. 
Diameter of the tubes 0.06 mm. 
Distance between the fascicles 0.12-0.20 mm. 
Measurements. Width of the f asc i c les 0.10 mm. 
Width of the zoarium 0.8 mm. 
Affinities. — In the number of the tubes to the fascicles this species approaches 
Idmonea arcuata; it differs from it in the separation of the fascicles which is less 
than 0.20 mm. and in the rectilinear form of its zoarium. 
In the zoarial margins of the basal lamellae the species resembles Idmonea 
petri D’Archiac, 1846, but differs from it in the ensemble of its much smaller 
micrometric. dimensions and in its claviform zoarium. 
Occurrence. — Midwayan (Clayton limestone) : One mile west of Fort Gaines, 
Georgia (rare). 
Cotypes. — Cat. No. 65246, U.S.N.M. 
IDMONEA MAGNA, new species. 
Plate 137. figs. 1-18. 
Description. — The zoarium is large , dichotomously branched, sometimes retic- 
ulated, enlarged at the bifurcations, triangular, higher than wide in transverse 
section. The fascicles are quite salient, oblique, alternated on each side of the 
median crest and quite distant from it ; they bear four to eight zooecia. The tubes 
are visible, convex, almost all equal. The basal lamella is striated transversally, 
flat, somewhat convex or a little concave; the tubes are often visible; the zoarial 
margins are very narrow and somewhat thickened. 
M easurements . — 
Diameter of the tubes 0.34 mm. 
Distance between the fascicles 0.70 mm. (0.40-0.80 mm.). 
Width of fascicles 0.24-0.30 mm. 
Zoarial width 1.40-2.00 mm. 
Variations. — The number of the tubes to the fascicle is more often four or 
five (figs. 2, 3) ; at the bifurcations it is larger (eight) (fig. 9). The basal lamella 
is convex (fig. 5) or concave (fig. 10) ; the two forms are visible on the same frag- 
ment (fig. 8). The fascicles of the reticulated branches are closer (fig. 4). At 
the bifurcations the fascicles are transverse (fig. 2), if the branches rest in the 
same plane; when the branches are twisted and are opposite, the fascicles remain 
distinct and alternate (fig. 9). On well-preserved specimens the tubes are visible 
on the basal lamella (fig. 6). The incomplete zooecia in the process of formation 
are quite numerous (fig. 10) and give a very crowded appearance. 
In transverse section (fig. 18) the zoarium is triangular and the zooecia are 
polygonal; but seen from the interior, by abrasion of the basal lamella, they are 
round (fig. 15). The longitudinal section (fig. 11) is normal; the tubes grow 
