NO. 2 OSBURN: eastern pacific BRYOZOA CHEILOSTOMATA 295 



Because of the simple nature of the aperture and operculum the 

 genus appears to belong with the more primitive Ascophora and the dorsal 

 radicular processes suggest the family Petraliidae. 



Robertsonidra oligopus (Robertson), 1908 

 Plate 34, figs. 9-11 ; Plate 35, fig. 1 



Schizoporella oligopus Robertson, 1908:292. 

 Not Schizoporella oligopus Waters, 1918:18. 



Zoarium encrusting, white to brick red, loosely attached in a single 

 layer by short tubular dorsal processes. The zooecia show a remarkable 

 degree of variation in dimensions, form of aperture and avicularia, often 

 within the same colony. The zooecia usually range between 0.50 and 

 0.75 mm long by 0.35 to 0.50 mm wide, very distinct at all ages. The 

 front is ventricose and consists of a thin pleurocyst with a single row 

 of areolar pores (rarely a few additional ones) and is thickly decorated 

 with small papillate tubercles ; the areolar pores and tubercles are 

 usually not visible until the thick ectocyst is removed. There is a small 

 rounded umbo, centrally placed proximal to the aperture, often wanting. 

 The primary aperture varies in form and size; semicircular and evenly 

 rounded distally and on the sides, the proximal border broadly arcuate, 

 or with a broad and very shallow sinus, or often nearly straight, all 

 within the same colony; in the infertile zooecia the aperture measures 

 0.14 to 0.16 mm long by 0.16 to 0.20 mm wide, while that of the 

 ovicelled zooecia measures 0.20 to 0.22 mm in width. The operculum 

 is well chitinized, colored like the frontal ectocyst, with a narrow sclerite 

 slightly within the border, the muscle attachments on the sclerite; the 

 proximal border is thinner and without a sclerite. The peristome is 

 low or wanting proximally, somewhat elevated distally, thin and smooth, 

 and the operculum is fully exposed to view. No spines, no cardelles and 

 no lyrula. There are numerous uniporous septulae scattered irregularly 

 over the lateral and distal walls. 



Moderately large avicularia occur sporadically, sometimes abundantly 

 but often wanting from considerable areas; situated on one side near 

 the aperture (rarely on both sides), with a large chamber which is 

 considerably elevated and provided with areolar pores and tubercles 

 similar to those on the front. The mandibles are of two kinds, the 

 usual ones triangular with a strongly decurved tip ; the others, replacing 

 the usual form, are elongate (as much as 0.40 mm), and rarely inter- 

 mediate conditions occur. The mandibles are heavily chitinized, with 

 a rounded lucida and are directed more or less sideways ; there is complete 

 hinge-bar. 



