NO. 3 OSBURN : EASTERN PACIFIC BRYOZOA — CYCLOSTOMATA 781 



Zoarium small, encrusting on stones and shells, white and glistening. 

 The zooecia are small, 0.40 to 0.50 mm long by 0.25 to 0.30 mm wide; 

 the front considerably inflated and delicately granulated; the areolar 

 pores are few in number and conspicuous only in the younger stages. 

 The zooecia are very distinct, with deep separating grooves which never 

 become filled even in the oldest and most complete stages of calcification. 

 The aperture is semicircular, 0.13 mm wide by 0.10 mm long, the 

 proximal border straight or slightly arcuate and without any indication 

 of a lyrula. The primary peristome is low and thin and there are no 

 spines, even in young marginal zooecia. There is a rounded median 

 avicularium with a prominent bulbous chamber which covers nearly 

 half of the frontal area proximal to the aperture. In advanced calcifica- 

 tion, the frontal becomes very thick, extends around the sides of the 

 aperture, and sometimes forms a small umbonate process on the top of 

 the avicularian chamber. 



The ovicell at first is prominent, 0.24 mm wide by 0.22 mm long, 

 but soon becomes more or less immersed, thick-walled and granulated 

 like the frontal. 



The species resembles P. concinna (Busk), but the smaller dimen- 

 sions, the proportionally larger avicularian chamber, and the distinct 

 separation of the zooecia at all stages clearly distinguish it. It is known 

 from the British Islands (Norman and Hincks), and Nordgaard rede- 

 scribed it as P. alba from the North American Archipelago. 



Point Barrow, Alaska, Arctic Research Laboratory, at 216 to 522 

 feet, G. E. MacGinitie, collector, rather common. 



Mucronella microstoma (Norman), 1868 

 Plate 81, fig. 10 



Lepralia microstoma Norman, 1864:87. 



Mucronella microstoma, Hincks, 1880:370. 



1 Mucronella microstoma, O'Donoghue, 1923:46. 



This species bears much resemblance to M. labiata, but it is smaller; 

 the primary aperture 0.13 mm wide by 0.10 long, semicircular, straight 

 on the proximal border with a wide lyrula which is low and without 

 denticles at the corners. The secondary aperture also is much smaller 

 than in labiata, wider than long, and the peristome is more or less 

 tubular, rising high on the proximal border and often continued into a 

 central point. The dietellae are small and numerous. There are 2 to 4 

 short stout oral spines on the distal border of the aperture, the two 

 median ones the largest. 



