268 University of California Publications in Zoology. [Vol.4 



aperture occupying the whole of the front, closed by a transpar- 

 ent, rather delicate membrane ; at each anterior angle a blunt 

 chitinous spine; operculum a simple, curved, semi-circular bar of 

 chitin (fig. 19a). Ooeciawantiflg. 



This species is found only in moderate quantity on the shores 

 of Alaska, Puget Sound, and California. It is not so abundant 

 in the dredgings as the next species described, M. serrata Hincks, 

 nor have I ever found it on the shore seaweed in anything like 

 the quantity in which that species is commonly obtained. Until 

 one has acquired experience there is no difference discernible to 

 the naked eye between these two species and the same habit 

 sketch (fig. 20) answers for both. 



45. Membranipora serrata Hincks. 



PI. 16, figs. 20, 21, 21a. 

 Memhranipora wemhranacea form serrata Hincks, 1882, p. 469. 



Zoaria consisting of circular patches incrusting seaweed (pi. 

 16, fig. 20), especially the fronds of the giant fucus of this coast; 

 colonies frequently numerous and by their coalescence covering 

 the entire surface on which they are growing. Zocecia quadrang- 

 ular, oblong, alternate, arranged in lines radiating from the cen- 

 ter of the colony (fig. 20) ; aperture occupying the Avhole of the 

 front, closed by a membranous front wall ; margins raised, with a 

 calcareous, crenated, inner rim, some of the crenations growing 

 long and forming distinct denticles; usually a rather long den- 

 ticle, cl., in the middle of the proximal rim of each zoa?cium, and 

 often several at irregular intervals on the lateral margins ; oper- 

 culum (fig. 21a) curved, its margin strengthened by a chitinous 

 rib, the proximal ends of which are drawn toward each other so 

 that a line joining them is shorter than one cutting the rim of the 

 operculum above its proximal extremities, i.e., the curve of the 

 operculum is greater than a half circle. At each distal angle a 

 short spine. Avicularia and oeEcia W' anting. 



Hincks gave but a short description of this species without 

 figure, considering it a form of M. memhranacea. From the ex- 

 amination of a large quantity of material from various localities 

 it seems justifiable to consider this a distinct species. The ser- 



