100 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



retractor muscle of the polypide inserted on the median axis of the zooecium assures 

 the symmetry of the opesium; moreover, there is no ocivell. 



The spinous processes are of the same nature as those of the genera Hemisep- 

 tclla Levinsen, 1909, and Cupularia Def ranee, 1821. but they are very inconstant; in 

 fact, they have not been noted on the recent specimens, nor are they very frequent 

 on the fossils. On the contrary, the serrate denticle is rarely found in the fossil 

 examples. 



ACANTHODESIA SAVARTII Audouin, 1826.' 



Plate 21, figs. 2-4. 

 1826. Flustra savartii AUDOUIN, Explication sommaire cles planches de 1'Egypte et de la Syrie, 



in Savigny's description cle FEgypte, Histoire Naturelle, vol. 3, Paris, p. 240, pi. 10, 



fig. 10. 

 3907. Mcmbranipora savartii CANU, Bryozoaires des terrains tertiaires des environs de Paris, 



Annales Paleontologie, vol. 2, p. 6, pi. 1, fig. 1. 

 ]908. Mcm'branipora savartii CANU, Ic-onographie des Bryozoires fossiles de 1'Argentine, 



Anales del Museo Nacional de Buenos Aires, vol. 17, p. 252, pi. 2, figs. 5, 6. 

 3909. Nembrampora savartii WATERS, Reports on the Marine Biology of the Sudanese Red 



Sea, XII, Journal Linnean Society, London, vol. 31, p. 137, pi. 11, figs. 8-13. 



1913. Membranipora savartii WATERS, Marine Fauna of British East Africa and Zanzibar. 



Bryozoa, Cheilostomata, Proceedings Zoological Society London, 1913, p. 486. 



1914. Membranipora savartii OSBURN, The Bryozoa of the Tortugas Islands, Florida, Publi- 



cation Carnegie Institution of Washington, No. 182, p. 1941. 



ho= 0.26-0.32 mm. . [Z,2=0.36-0.44 mm. 



mm. Zo0eCli >=0.18-0.36 mm. 



Variations. The American forsil specimens are unilamellar and incrust other 

 bryozoa, chiefly the Cellopores. After rubbing away the zoarial surface we have 

 observed only one distal septula. The micrometric variations between one specimen 

 and another are considerable. Our specimens correspond exactly to those from the 

 environs of Paris, and are almost the same as those from Egypt and from the Eng- 

 lish Crag, but they differ a little from examples dredged in the recent seas. We 

 have figured the extreme forms, but our fossils show all of the intermediate stages. 



The opesial denticle is rarely observed on the fossils on account of its very great 

 fragility. Nevertheless, these denticles are very clearly preserved on a specimen 

 from the Vicksburgian of Jasper County, Mississippi (fig. 4). 



The vigor and resistance of this species is extraordinary. Quite cosmopolitan 

 in the existing seas, it was present even in the Eocene seas. Unfortunately, we are 

 still ignorant of its larval system. It appears sensitive to bathymetric variation, 

 implying an elementary hydrostatic system, and the absence of powerful means 

 of oxygenation. 



Occurrence. Vicksburgian ("Chimney rock" of Marianna limestone) : Three 

 miles southeast of Vosburg, Jasper County, Mississippi (rare) ; 1 mile north of 

 Monroeville, Alabama (very common). 



1 Snvigny having become blind, the explanation of his plates was prepared by Audouin in 126 

 Savigny's plates were published in 1812. 



