NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
481 
This genus differs from Smittina only in the calcification; the tremocyst re- 
places the pleurocyst. There are, however, some other secondary differences. The 
mandible of the avicularium is semicircular; it is generally (but not universally) 
triangular in Smittina. The lyrula is wanting in Porella; however, Porella majus- 
cula Nordgaard, 1905, Porella minuta Norman, 1868, Porella irregularis , new species, 
and Porella coronata , new species, have an apparent lyrula. 
The median avicularium is quite variable. In the same species it may be quite 
salient or buried in the peristomie, according to the thickness of the tremocyst; 
these variations make the determination of the species very difficult. Some species 
have the avicularium always salient; others, on the contrary, have an invisible 
avicularium, and only abrasion of the surface will reveal it. 
Historical. — The genera Smittina and Porella are two quite natural genera 
which have been admirably described by Hincks, but of which he has not given the 
exact limits, occasioning thus some hesitation among collectors. Waters, in 1900, 
recognized the necessity of making two groups in the genus Porella , according to 
the nature of the mandible. The first group is very homogeneous and is the genus 
Porella as Levinsen limited it in 1909, and as we now understand it. The second 
group is less homogeneous, for the presence of a lucida on the mandible does not 
appear to correspond to an important physiologic function. In 1909 Levinsen 
thought Porella saccata Busk, 1856, belonged to a distinct genus; this is also our 
opinion and we have created the genus Cystisella. Likewise he thought that Porella 
acutirostris might serve as genotype for another genus. On the other hand, we 
think that Porella margaritigera Quoy and Gaymard, 1833, may belong to a dif- 
ferent genus, for which we propose Ilippadenella (see p. 402). Still further studies 
are necessary. P almicellaria is a zoarial subgenus which may be conveniently pre- 
served. 
PORELLA OBLIQUA, new species. 
Plate 14, fig. 17. 
Description. — The zoarium incrusts shells. The zooecia are small, a little 
elongated, little distinct; the frontal is little convex and formed by a tremocyst with 
large pores. The apertura is semilunar ; the peristome is smooth, thin, more or less 
salient. The ovicell is hvperstomial and opens into the peristomie ; it is formed of 
a porous area surrounded by a smooth collar. The median avicularium is trian- 
gular, salient, oblique; its orifice is disposed laterally. Often tivo adventitious 
avicularia hide the frontal more or less. 
A-ffinities. — This small species is quite fragile and of a disconcerting irregu- 
larity. The very special arrangement of the avicularium, which opens on the side 
in a very oblique manner, is characteristic of the species. The rarity of the speci- 
mens has not permitted us to make a careful study, but we have thought it worth 
while to call the species to the attention of collectors. 
Occurrence— Lower Jacksonian: Three and one-half miles southeast of Shell 
Bluff post office, Georgia (rare). 
Holotype. — Cat. No. 63852, U.S.N.M. 
55899— 19— Bull. 106 31 
