NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
99 
In its exterior aspect this species is close to Acanthodesia (M embranipora) 
savartii Audouin, 1826, but it differs in its smaller micrometric dimensions, its 
tubular zoarium, and in the absence of an opesial denticle. In considering this 
form of zoarium it is necessary to admit that in order to attain such a shape there 
is necessary, first, an alga very soft and incapable of breaking the armor formed 
about it by the bryozoan, and, second, either the base or the joint of the incrusted 
alga has reached its greatest resistant growth, is incapable of further growth, 
or is dead. We are without means of elucidating such problems, but one must 
admire the remarkable selective instinct of the larva which would always choose 
an identical substratum and could anticipate its nondevelopment. We will dwell 
many times on these marvelous larval activities in the course of this work. It is 
greatly to be desired that their study will be undertaken by the young zoologists. 
Occurrence. — Vicksburgian (“Chimney rock” of Marianna limestone) : 1 mile 
north of Monroeville, Alabama (very common) ; 2-1- miles north of Millry, Washing- 
ton County, Alabama (rare). 
Cotypes. — Cat. No. 64230, U.S.N.M. 
MEMBRANIPORINA ARCANA, new species. 
Plate 80, figs. 8-11. 
Description. — The zoarium is a fragile, free network. The zooecia are very 
elongate, little distinct, nearly elliptical ; the mural rim is very thin at the top, en- 
larged below, curved, almost smooth. The opesium is large, elliptical, entire. There 
is no calcified dorsal face. 
fZs=1.00 mm. 
M easurements. — Zooecia 
fe=0.40 mm. 
A-ffinities. — This species is a true mystery. As it is very fragile and imperfectly 
calcified we would be justified in classifying it in M embranipora — that is to say, in 
the Membranacea group. But we have discovered a sort of ovicell (?) situated in 
the lower part of the zooecium and deforming its proximal part. This problematic 
cavity has never been observed before. We are thus compelled to class it in the 
genus Membraniporina , with species insufficiently studied or poorly understood. 
Occurrence— Vicksburgian (“ Chimney rock ” of Marianna limestone) : One 
mile north of Monroeville, Alabama (rare). 
Cotypes. — Cat. No. 64231, U.S.N.M. 
ACANTHODESIA, new genus. 
Greek: Acanthodes, full of bristles. 
No ovicell. The opesium is garnished laterally by small spinous processes 
and interiorly by a serrate denticle. Fifteen tentacles. 
Genotype. — Acanthodesia ( Flustra ) savartii Savigny- Audouin, 1826. 
Range. — Lutetian-Becent. 
Another species of this new genus is M embranipora Umosa , Waters, 1908. 
Waters classified the genotype in the same group as M embranipora Mans Hincks, 
1885. This is incorrect, since Flustra savartii is deprived of opesiules, and its large 
