240 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
As they are a form of floating life, the Lunulltes are subject to variations 
from hydrostatic pressure quite as much as the more mobile fish. To avoid this 
the zoarium increases or diminishes its volume by the aid of parietal muscles at- 
tached to the ectocyst; it also increases its volume by the development of tuberosities 
on its noncelluliferous face or by special tubules on the same face. This organi- 
zation, infinitely varied in details, permits the animal to be assured of its existence 
under a rather constant depth of water and to avoid the great strain of hydrostatic 
pressure. 
Geological distribution. — Many Cretaceous Lunulltes are possessed of opesiular 
indentations and endozooecial ovicells and consequently ought to be classed in the 
genus Lunularia. The figures of the authors are often inaccurate and it is necessary 
to have recourse to direct observation. By this means we have established that the 
following species are true species of Lunularia: 
Lunulltes plana D’Orbigny, 1852. 
Pavolunulites elegans D’Orbigny, 1852. 
Pavolunulites costata D’Orbigny, 1852. 
Lunulltes munsteri Hagenow, 1851 (=L. patelliformis Marsson, 1877). 
Lunulltes radlata Lamark, 1816. 
Lunulltes urceolata Cuvier, 1822. 
Lunulltes beisseli Marsson, 1887. 
Lunulltes salebrosa Marsson, 1887. 
Lunulltes goldfussi Hagenow, 1851. « 
/Structure of the ovlcell. — The ovicell is truly endozooecial and closed by the 
opercular valve. Because of the elevation of the zooecial axis it forms only a small 
distal cavity underneath the operculum and is situated on an enlargement of the 
distal zooecium (pi. 83, fig. 2). On account of its minuteness the ovicell has 
escaped observation, but with a little attention it is easy to discover it and to do 
so rather frequently. 
Zoarial growth. — The zoarial growth is very different from Trochopora , which 
increases in size by successive disks. It is made by pluricellular segments radiating 
from the zoarial center (pi. 83, fig. 8) or from a bifurcation (pi. 13, fig. 12). This 
is quite visible chiefly on the fractures (pi. 83, fig. 7), but it may also be seen on 
the sections (pi. 83, fig. 10). 
LUNULARIA REVERSA Ulrich, 1901. 
Plate 1, figs. 17-19. 
1901. Lunulltes reversa Ulrich, Maryland Geological Survey, Eocene, p. 217, pi. 60, 
figs. 19, 20. 
Description. — Zoarium flabellate (perhaps originally discoid or depressed coni- 
cal). Zooecia subquadrate or pentagonal, usually widest in the anterior half, 
arranged in rather irregular radiating lines, about six in 2 mm., with an impressed 
line separating the rows ; area depressed, its surface grano-lineate. Margin strongly 
raised, thick, and straight or slightly concave across the posterior end and much 
