NO. 3 OSBURN : EASTERN PACIFIC BRYOZOA CYCLOSTOMATA 775 



diameter, spreads across a frond of Porella compressa, the edges extend- 

 ing beyond the side of the frond but remaining unilaminar. The zooecia 

 are large, 0.65 to 1.0 mm or more in length by 0.45 to 0.60 mm in 

 width, arranged quincuncially. In the infertile zooecia the opesia occu- 

 pies practically all of the frontal area; the side walls are very thin and 

 finely granulated. Large interzooecial avicularia are scattered over the 

 zoarium, usually at the beginning of the new series of zooecia. The 

 chamber is nearly as long as the normal zooecia, 0.50 to 0.65 mm, but 

 narrower, about 0.40 mm in width. The mandible, 0.35 to 0.50 mm 

 long by 0.20 to 0.26 mm wide, has the shape of a gothic arch, the sides 

 parallel and the pointed tip strongly decurved to fit inside the recurved 

 tip of the rostrum. Minute avicularia (mandible 0.06 to 0.10 mm 

 long) are situated at the distal zooecial comers and often there are one 

 or two additional ones, with the mandible directed forward instead of 

 backward as in H. nigrans; occasionally there are similar small avicularia 

 on the side walls and in these the position of the mandible is reversed, 

 pointing backward. Near the center of the colony there are 5 or 6 short 

 stout spines and one or two of these may be present on the zooecia over 

 the whole zoarium. 



The endozooecial ovicells are similar to those of nigrans, but the cover- 

 ing transverse ridges are much less developed and there is always a pair 

 of the small avicularia distally placed on either side of the ovicell with 

 the mandible directed forward. 



The species is evidently related to H. nigrans, but the differences in 

 the avicularia and the cover of the ovicell and the presence of spines 

 appear sufficient to give it specific rank. H. nigrans also has large inter- 

 zooecial avicularia, but they are short with a short triangular mandible. 



Type, U. S. Nat. Mus. no. 11048. 



Type locality, Point Barrow, Alaska, 453 feet. Numerous colonies 

 were obtained by Prof. G. E. MacGinitie at depths ranging from 216 to 

 522 feet. 



Amphiblestrum trifolium (S. Wood), 1850 

 Plate 81, fig. 2 



Flustra trifolium S. Wood, 1850:20. 

 Membranipora solida Packard, 1867:272. 

 Membranipora Flemingii var. solida, Verrill, 1879:29. 

 Membranipora trifolium, Hincks, 1880:167. 

 Membranipora trifolium, Whiteaves, 1901 :97. 

 Membranipora trifoliu/n, Osburn, 1912:279. 

 Amphiblestrum trifolium, Osburn, 1923:9; 1932:9; 1933:26. 



