240 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



As they are a form of floating life, the Lunulites 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 Lunulites 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: 



Lunulites piano, D'Orbigny, 1852. 



Pavolunulites elcgans D'Orbigny, 1852. 



Pavolunulites costata D'Orbigny, 1852. 



Lunulites munsteri Hagenow, 1851 ( = L. patclliformis Marsson, 1877). 



Lunulites radiata Lamark, 1816. 



Lunulites urceolata Cuvier, 1822. 



Lunulites beisseli Marsson, 1887. 



Lunulites salebrosa Marsson, 1887. 



Lunulites goldfussi Hagenow, 1851. 



Structure of the ovicell. 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 Trocfwpora, 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 REVEESA Ulrich, 1901. 



Plate 1, figs. 17-19. 



1901. Lunulites rercrsa ULBICH, Jinn-land Geological Survey, Eocene, p. 217, pi. GO, 

 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 



