MARYLAND GEOLOGICAL SURVEY 413 



Membranipoka fistula n. sp. 

 Plate CXII, Fig. 5. 



Description. — Zoarium so far as observed forming small, hollow, sub- 

 cylindrical stems, about 1.5 mm. in diameter, composed of twelve to 

 fifteen longitudinal rows of zooecia. Walls about as wide as the zooecial 

 openings, obtusely carinate, the carinas between the ends of the cells 

 high and bent forward so as to impart a slightly imbricating appearance 

 to successive cells. Opesial opening elongate elliptical; immediately 

 behind it a minute pore is occasionally noticeable. Measuring longitu- 

 dinally about 8 zooecia in 5.0 mm. No avicularian nor vibracular cells 

 observed. 



Though numerous allied species, chiefly among those assuming the 

 bifoliate mode of growth, are known, none is sufficiently close to cause 

 us to think that M. fistula may be but another zoarial phase of it. For 

 the present then we consider ourselves justified in proposing a new name 

 and further in assuming that the zoarial characters above described are 

 normal for the species. The cylindrical stems of Flustrellaria texturata 

 Keuss, from the Miocene of Austria are very similar, the principal differ- 

 ences being that their apertures are shorter and rounder. With a large 

 series of specimens M. -fistula may prove to be but a variety. 



Occurrence. — St. Mary's Formation. St. Mary's Eiver. 



Collection. — Maryland Geological Survey. 



Genus AMPHIBLESTRUM Gray. 

 x4.MPHIBLESTRUM CONSTRICTUM n. Sp. 



Plate CXY, Figs. 6, 7. 



Description.- — Zoarium forming crusts of small extent upon shells, the 

 types growing on a Pecten. Zocccia arranged in rather irregular rows, 

 6 to 8 in 3 mm. Aperture subovate, more or less constricted in front 

 of the midlength, enclosed a sloping and finely striated border, 

 widest posteriorly. Frontal lamina very little developed forming 

 the sloping and transversely striated border just mentioned. 

 Ovicells abundant, large, moderately convex, the middle portion distin- 

 guished by being minutely pitted or porous. Avicularia of moderate 



