334 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 14 



cyst with numerous small pores, considerably inflated and with or without 

 the small umbo which is present in typical specimens. The aperture is 

 semicircular, about 0.18 mm wide, the proximal border straight with a 

 narrow, deep, slit-like sinus. The peristome is unusually thin and low. 

 No spines, no avicularia. 



The ovicell is very prominent, longer than wide, smooth, imperforate. 



Hincks and O'Donoghue have reported the species for several lo- 

 calities in British Columbia, and Robertson recorded it from San Pedro, 

 California. 



Hancock Station 328, Chatham Bay, Cocos Island, Costa Rica at 

 14 fms. It is a very widely distributed species, but appears to be rare 

 in the Eastern Pacific area. There are also specimens collected by Miss 

 A. E. Blagg off Lighthouse Point at the entrance to Monterey Bay, 

 California. 



Arthropoma circinata (MacGillivray), 1868 

 Plate 38, fig. 4 



Lepralia circinata MacGillivray, 1868 :9. 

 Schizoporella circinata. Busk, 1884:166. 

 Schizoporella circinata, Hincks, 1885 :253. 



Zoarium encrusting, unilaminar. Zooecia of moderate size, 0.40 to 

 0.50 mm long by 0.25 to 0.35 mm wide, irregularly hexagonal, very 

 distinct with deep separating grooves ; the frontal high and evenly arched, 

 smooth or somewhat reticulate in older zooecia, with conspicuous pores 

 and a small, smooth central area. Proximal to the aperture is a thin, 

 arcuate, umbonate process with a concavity on its distal side forming 

 a shallow pouch, but there is no other appendage or decoration. The 

 primary aperture is semicircular, about 0.12 mm wide, the proximal 

 border straight with a slit-like sinus. The operculum is thin, conforming 

 to the aperture and sinus, with a narrow bordering sclerite and the 

 muscle attachments removed from the border. The peristome is low 

 and thin, with 6 short, stout spines which are often little more than 

 tubercles. No avicularia. 



The ovicell is prominent, smooth and imperforate, 0.20 to 0.25 mm 

 wide and usually a little longer than wide, not closed by the operculum. 



The species is similar in appearance to A. cecili, but it is much 

 smaller, and the smooth central area of the front is larger, the umbonate 

 process is thin and arcuate, and there are vestigial oral spines. 



Known from Australia, New Zealand and Tristan da Cunha. 



