1908] Robertson. — Incrusting Bryozoa. 259 



36. Membranipora circumclathrata Hincks. 



PI. 14, figs. 1, 2. 



Memhranipora cimtmclathrata Hincks, 1881, vol. 8, p. 131, pi. 5, 

 fiff- 1. 



Zoaria forming small, delicate, spiny, circular incrustations 

 on pebbles or seaweed. Zooecia alternate, elongated, narrowed 

 below and separated from eaeb other by areolations (pi. 14, fig. 1 

 are.) ; aperture oval, occupying at least two-thirds of the front, 

 and closed by a thin membrane; margin of aperture narrow, cal- 

 careous, raised, slightly crenate on the inner rim ; surrounded by 

 a large number of spines, some stiff and flaring, others flexible 

 and curving inwards ; two, frequently four spines, fl. sp., at the 

 distal margin, stiff, flaring outward and upward; below these, two 

 other stiff spines, st. sp., extending either straight forward or in- 

 clining slightly inward ; on the lower three-fourths of the aper- 

 ture, six or eight flexible spines, sp., inclining inward across the 

 aperture and frequently meeting in its middle line. A sessile 

 avicularium, av., just below the margin, calcified lines radiating 

 from it to the edge of the zooecium forming large, thin walled 

 spaces which with the areolated spaces between the zooecia char- 

 acterize this species. Omcia (fig. 2, oe.) smooth, with stride radiat- 

 ing from the proximal or oral margin to the distal margin ; a rib 

 extending across the front a slight distance above the oral edge ; 

 each occcium covering the calcified portion of the zooecium next 

 above having the avicularium, av., of that zooecium perched ap- 

 parently upon its summit ; these ooecial avicularia usually larger 

 than the ordinary ones on the front of the zooecia, with tapering 

 mandible extending upward on one side of the aperture ; below 

 each ooecium a pair of stiff spines, inclining slightly toward each 

 other. 



In his description of this species, Hincks ('81) speaks of 

 finding a line of small holes on the margin of the aperture which 

 he suggests may indicate the position of as many spines. That 

 observer probably possessed only imperfect, nnitilated specimens. 

 The colonies here described were in fine growing condition and 

 the margins of the apertures as shown are bordered with spines, 



