306 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
V aviations . — The number of the costules is rather variable; they are arranged 
horizontally at the top and radially below. The lumen pores are sometimes visible 
(fig. 9) ; they are irregularly placed and never prominent. 
There are sometimes two distal avicularia (fig. 9) ; their points touch above 
the aperture on the median zooecial axis; there are some zooecia unprovided with 
avicularia (fig. 9). 
In the interior (fig. 10) the proximal border of the aperture is bordered by a 
thick collar. The avicularia are interzooecial. 
The margins of the costules are invisible in tangential section (fig. 12) ; the 
lumen pores are visible, but not the lumen itself. In figure 12 the lacunae are 
seen to be surrounded by a polygonal network, which indicates the boundary of 
the primitive spines. 
Affinities . — This species differs from Metracolposa brevis in the presence of 
large distal avicularia and in its greater zooecial length, and from Metracolposa 
robusta in its larger zooecia and its much larger and more numerous avicularia. 
On account of the size of its zoarium this is an easily recognized fossil. 
Occurrence .- — Middle Jacksonian: Wilmington, North Carolina (common); 
near Lenuds Ferry, South Carolina (common). 
Holotype.— Cat. No. 61033, U.S.N.M. 
METRACOLPOSA ROBUSTA Canu and Basslcr, 1917. 
Plate 43, figs. 1-7. 
1917. Metracolposa robusta Canu and Basslee, Synopsis of American Early Tertiary 
Clieilostome Bryozoa, Bulletin 96, United States National Museum, p. 35, pi. 3, fig. 6. 
Description .- — The zoarium is free, bilamellar, large (1 to 2 centimeters in 
width), solid, robust. The zooecia are distinct, elongated, elliptical; the frontal 
is somewhat convex ; the costules are transverse at the top, radial below ; they num- 
ber from seven to nine pairs and are separated by four or five large lacunae; 
the lumen pores are small and irregular. The aperture is transverse, semielliptical 
with a straight or slightly convex proximal border; it is bordered distally by a 
very thin, incomplete peristome. The ovicell is large and deeply embedded in the 
distal zooecia, elongated, salient, convex, decorated in front with a deltoid Carina; 
it opens above the aperture and probably was closed by the operculum when it 
opened. The aperture of the ovicelled zooecia is larger. A small, triangular distal 
avicularium is placed either on the right or left of the aperture. 
M easurements . — Aperture (ordinary) < 
Aperture (ovicelled) 
Zooecia 
7* < 2 = 0.10 mm. 
la= 0.20 mm. 
ha=0. 12-0.14 mm. 
Ia= 0.28-0.30 mm. 
Lz— 0.96-1.00 mm. 
lz= 0.44-0.46 mm. 
Variations . — The zooecial width is rather variable; there are some zooecia 
which measure 0.51 mm., in which case the costules are radially arranged (fig. 2). 
