26 BULLETIN 16 5, UNITED STATES NATIONAL MUSEUM 



Description. — The zoarium is encrusting, growing upon shells, 

 echinoids, and other Bryozoa. The zooecia are distinct, separated 

 by a deep furrow, very elongated, oval, generally enlarged at the 

 base ; the mural rim is thin, rounded, with six pairs of hollow spines ; 

 the opesium is large, entire, of the same form as the zooecium. There 

 are a distal septula and six lateral pairs opened widely at the bottom 

 of the zooecium. The ovicell is hyperstomial, not closed by the oper- 

 culum, small, globular, smooth. Above each zooecium there is an 

 interzooecial avicularium arranged longitudinally, with its beak 

 somewhat enlarged and oriented toward the base. The orifice is 

 symmetrical and slightly narrowed toward the middle. 



Measurements. — 



\Lz= 0.48-0.57 mm. ~ . \ho= 0.39-0.43 mm. 

 Zooecium \ 1 „ Opesium, A1D noi 



1 7s = 0.27-0.3 mm. l [lo = 0.18-0.21 mm. 



25-28 zooecia is 4 sq. mm. 



Variations. — The micrometric measurements vary from one zoar- 

 ium to another; the largest figures that we give above appear to be 

 the maximum. Rather rarely there is a very small gymnocyst below 

 the opesium. Elliptical and nonpyriform cells are not rare. We 

 have often observed regenerated zooecia. 



The ordinary avicularia appear to be oriented toward the aperture 

 of the proximal zooecium. On the contrary, the avicularia with gym- 

 nocyst and transformed into zooeciules always have their beak 

 oriented distally. 



The ovicells replace the avicularia. The latter are quite constant, 

 and their position is always the same except when the irregular- 

 ities of the substratum disarrange the gemmation. When they are 

 absent between the nonovicelled zooecia, it is always easy to dis- 

 cover them on the interior of the distal zooecium. We have many 

 times observed this curious phenomenon both on the fossil forms and 

 on Recent specimens. It is inexplicable. 



Gabb and Horn's figure, lacking the areal spines, is incomplete, but 

 the interpretation of Ulrich and Bassler, 1907, is correct, all the other 

 characters being identical. We are not certain that our generic 

 reference is exactly correct. Membranipora humiliata Brydone, 1910, 

 and M. anterides Brydone, 1910, both from the English Senonian, 

 have an analogous structure. 



Occurrence. — Vincentown limesand : Mullica Hill (Gabb and 

 Horn), Vincentown and near Blackwoodstown, N.J. (common); 

 Noxontown Millpond, Del. 



Plesiotypes.—U.S.'NM. Nos. 73869-73871. 



