NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
307 
The avicularia are rather rare; their point is directed toward the median 
axis of the zooecia above the aperture; sometimes they are vertical; when well 
preserved they have a calcareous pivot. 
In the interior (fig. 7) the ovicell is seen to occupy the larger part of the 
cavity of the distal zooecia — a condition which is better visible on the vertical 
section (fig. 5). This section also permits one to understand the movements of the 
operculum. This structure closes the ovicell at the moment of the passage of the 
eggs and during the evolution of the embryos. After the expulsion of the latter 
it closes only that zooecium in which the aperture has preserved its distal but 
deeply buried margin. 
In thin section the zooecial walls are thin, and composed of scattered olocvstal 
elements. The avicularia appear to be interzooecial. 
It may be that the costules are not hollow, for the lumen has never been 
observed. 
The two lamellae forming the zoarium are inseparable. 
There are four or five lateral septulae (fig. 6). 
Affinities. — Metracolposa robusta differs from Metracolposa brevis in the pres- 
ence of an avicularium and in its smaller zooecial width. 
It differs from Metracolposa grandis in its smaller general dimensions and its 
much smaller and rare avicularia, and from Metracolposa cylindrica in its bila- 
mellar zoarium. 
The zoarial dimensions are such that this is a good-sized fossil and easily 
recognized. 
Occurrence. — Middle Jacksonian: Wilmington, North Carolina (very com- 
mon) ; Eutaw Springs, South Carolina (rare). 
Cotypes. — Cat. No. 62586, U.S.N.M. 
METRACOLPOSA CYLINDRICA, new species. 
Plate 42, figs. 14-20. 
Description. — The zoarium is free, cylindrical , formed of six rows of zooecia. 
The zooecia are distinct, much elongated, elliptical; the frontal is convex; the 
costules are thick, numbering eleven or twelve pairs, separated by four or five 
rather large lacunae, without lumen pores. The aperture is transverse, semi- 
elliptical, with a straight or slightly concave proximal border. The ovicell is 
very large, embedded in the distal zooecium, elongated, prominent, convex, decorated 
in front with a deltoid carina; the ovicell opens above the aperture and probably 
was closed by the operculum. The avicularia are triangular and are placed in the 
junction angles of the zooecia; their point is turned toward the median axis of 
the zooecia. 
Measurements. — Aperture \ha= 0.10-0. 14 mm. Aperture fAa=0.17 mm. 
(ordinary) Ua= 0.16-0.18 imp. (ovicelled) I la=0.35 mm. 
„ . \Lz= 1.10-1.20 mm. 
/iooecici , 7 
