NORTH AMERICAN EARLY TERTIARY BRYOZOA. 99 



In its exterior aspect this species is close to Acanthodesia (Membranipora) 

 sarartii 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-i miles north of Millry, Washing- 

 ton County, Alabama (rare). 



C'otypes.CsLt. No. 64230, U.S.N.M. 



MEMBRANIPORINA ARCANA, new species. 



Plate SO, 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. 



. Zs=1.00 mm. 

 Measurements. Zooecia , _ ._ 



12OAO mm. 



Affinities. This species is a true mystery. As it is very fragile and imperfectly 

 calcified we would be justified in classifying it in Membranipora 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 inferiorly by a sen-ate denticle. Fifteen tentacles. 



Genotype. Acanthodesia (Fhistra] *</>, /,'///' Savigny-Audouin, 1826. 



Range. Lutetian-Recent. 



Another species of this new genus is M embranipora limosa Waters, 1908. 

 Waters classified the genotype in the same group as M cnibrunipora hians Hincks, 

 1885. This is incorrect, since Flusfru xiicartii is deprived of opesiules, and its large 



