NO. 2 OSBURN: eastern pacific BRYOZOA CHEILOSTOMATA 391 



considered, viz., the nature of the calcification of the frontal, the 

 presence or absence of frontal pores and ooecial pores, and the mode of 

 origin of the avicularia. 



1. The genotype of Porella is Millepora compressa Sowerby. Un- 

 fortunately when describing this genus Gray misidentified compressa with 

 Millepora cervicornis Pallas, which has somewhat the same growth form 

 but has a porous frontal (tremocyst). As late as 1920 Canu and Bassler 

 accepted cervicornis as the genotype of Porellaj but Bassler later (1935) 

 corrected the error. The genotype of Porella therefore has a bilaterally 

 symmetrical avicularian chamber, an imperforate frontal (except the 

 areolar pores), an imperforate ovicell and a very low, broad lyrula which 

 is so short as to be indistinguishable except when viewed from the interior 

 of the frontal, and no cardelles. The other species which may be allied 

 to co?npressa have the characters mentioned, but the frontal is usually 

 much smoother and is easily mistaken for an olocyst. Careful study of 

 incinerated specimens shows the secondary layer or pleurocyst. In most 

 of the species the frontal becomes very thick, so that the median avicu- 

 larian chamber and the ovicell are often completely buried beneath the 

 secondary crust and the areolar pores are often occluded. Frontal avicu- 

 laria also are sometimes present. 



2. The genotype of Smittina is Lepralia landsborovii Johnston which 

 agrees with Porella in the presence of a median bilaterally symmetrical 

 suboral avicularium, but in which the frontal is a tremocyst with numer- 

 ous pores, the ovicell is usually similarly perforated and the lyrula and 

 cardelles well developed. Usually the pores of the ovicell are well dis- 

 tributed, but in a few cases, bella Busk and retifrons, new species, they 

 are limited to 1 or 2 central pores and these may even be occluded in 

 final calcification. The tremocyst often becomes thick and the frontal 

 pores more or less infundibuliform. 



3. A third group, Smittoidea new genus, differs in having the frontal 

 a pleurocyst, with a median symmetrically developed suboral avicularium, 

 perforated ovicell and well developed lyrulae. 



4. Still a fourth group, Parasrnittina new genus, is easily distin- 

 guished by the pleurocystal front and the nature of the avicularia which 

 are variously distributed over the front but never median and suboral; 

 they take their origin from areolar pores on one side only. The lyrulae 

 and cardelles are well developed, though they are sometimes so hidden 

 by the overhanging peristome that dissection is required to reveal their 

 presence. The ovicell is variously perforated, sometimes by small pores 

 or by larger pores which often vary in size and form, or more rarely by 

 one to three central pores. 



