XORTH AMERICAN EARLY TERTIARY BRYOXOA. 481 



This genus differs from Xm/lthiii only in tin 1 calcification; the tremocyst re- 

 places the pleurocyst. There are, however, some other secondary differences. The 

 mandible of the aviculariuiu is semicircular; it is generally (but not universally) 

 triangular in Smittinn. The lyrula is wanting in Porclla; however. Porclla tf/}u*- 

 c-iila Xordgaard. 100.5. Porclla in inn fa Xorman. 1868, Porclla iri-<></iil<irix. new specie-. 

 and Porclla- roronata, 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. 



Hixfoi'icul. The genera Sin iff inn 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 lucicla on the mandible does not 

 appear to correspond to an important physiologic function. In 1909 Levinsen 

 thought Porclla xiiccatii Busk. 1S56, belonged to a distinct genus; this is also our 

 opinion and we have created the genus Cystisella, Likewise he thought that Por< //</ 

 acutirostris might serve as genotype for another genus. On the other hand, we 

 think that Porclla marc/aritif/era Quoy and Gaymard, 1833, may belong to a dif- 

 ferent genus, for which we propose IllppuJ< m II a (see p. 402). Still further studies 

 are necessary. Pol mi cell aria is a zoarial subgenus which may be conveniently pre- 

 served. 



PORELLA OBLIQUA. new species. 



Phite 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 hyperstomial 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 two adventitious 

 avicularia hide the frontal more or less. 



Affin'ttlcx. 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 <>l>li<j>i< 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). 



Holot!/p,'.('-A\. No. (13s.-)2. U.S.X.M. 

 .V,s99 lit Bull. 106 - 31 



