26 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 14 



Membranipora tenuis Desor, 1848 

 Plate 2, figs, 9 and 10 



Membranipora tenuis Desor, 1848:66. 

 Membranipora denticulata Busk, 1856 : 176. 

 Biflustra denticulata^ Smitt, 1873 :18. 

 Hemiseptella denticulata, Canu and Bassler, 1928:62. 

 A canthodesia denticulata, Hastings, 1930:707. 



The M. denticulata of Busk from the Pacific coast and the Biflustra 

 denticulata of Smitt from the Atlantic are definitely the same species and 

 the same as the tenuis of Desor, according to Dr. Anna B. Hastings {in 

 litt.) who has examined Busk's type material and a large number of 

 other specimens in the British Museum from various Atlantic and Pa- 

 cific localities. 



Zoarium encrusting anything that affords attachment, but most com- 

 mon on shells and stones; occasionally rising in free, bilaminate frills 

 similar to those of M. perfragilis but more delicate. The zooecia are 

 moderate in size, 0.45 to 0.50 mm long by 0.20 to 0.25 mm wide, but 

 often with a much wider range. The walls are moderately high and the 

 mural rim roughly and irregularly granulated ; usually there is no evi- 

 dence of a gymnocyst; the cryptocyst is well developed and exceedingly 

 variable, proximally it forms a broad shelf which extends more narrowly 

 along the sides and then becomes somewhat broader around the distal 

 border. Occasionally the shelf may fill in the whole basal half of the 

 opesia, or it may be limited to a narrow band ; the border of the shelf 

 bears very irregular and sometimes branched spinules, which may extend 

 laterally half way across the opesia or the border may be merely irregu- 

 larly serrate. The proximal shelf is flat and smooth except for a few to 

 many minute tubercles. The ectocyst is slightly brownish and often there 

 is a distinct brown line separating the mural rims. The operculum is 

 slightly more heavily chitinized than the frontal membrane and has a 

 thick brown border; width 0.13 mm. 



No ovicells, no avicularia, no spines, no dietellae. Small rough 

 processes may occur at the proximal corners, but these are extremely 

 variable in size and occurrence. The variations in zooecial size and form, 

 the extent of the cryptocyst, and the presence or absence of the tubercles 

 have led to the description of several supposed new species ; Membrani- 

 pora danica Levinsen, from Denmark, and Hemiseptella africana, H. 

 hexagonalis and H. grandicella of Canu and Bassler appear certainly to 

 belong under tenuis. 



