Polyzoa of the St. Lawrence. 221 



PORELLA, Gray. 



Porella Skenet, Ellis & Sol., form plana^ n. var. 

 (PL XIV. figs. 6.) 



Zoarium erect^ bilaminate, compressedj forming broad 

 flattish expansions, slightly divided into segments at the top, 

 which lie very much in the same plane, edged by a smooth 

 border composed of aborted cells ; surface smooth. Zocecia 

 large, arranged with much regularity in quincunx, elongate, 

 of about equal width throughout, subcylindrical, convex, 

 distinct, slightly tumid below the orifice (not suberect above), 

 surface shining, minutely granulated ; orifice (primary) semi- 

 circular ; adult orifice subquadrangular, peristome elevated, 

 so as to conceal the jTi'imary opening, destitute of spines, 

 lower margin slightly curved outwards, in the centre of it a 

 rounded avicularium, facing inward and just visible above the 

 margin ; on each side a stout erect process, somewhat enlarged 

 above and rounded at the top, where it curves slightly inward, 

 bearing immediately below the top, looking into the opening 

 of the cell, a small rounded avicularium, Ocecium ample, 

 rounded above, closely united to the neighbouring cells, often 

 subimmersed, surface granular, closed in front by a smooth 

 white porcellaneous plate, which stretches down into the cavity 

 of the cell. Circular avicularia often thickly scattered over 

 the zoarium. 



It is with some hesitation that I refer this form to P. 

 Skenei. There are many striking differences between the 

 two. In the first place there is a remarkable dissimilarity 

 in habit. In the present form the zoarium is much larger * 

 and more massive than in the normal P. Skenei, the segments 

 are broader, much in the same plane, and much less nume- 

 rous than in the other, and not so regularly truncate at the 

 extremity. Specimens present a flattish^ smooth, expanded 

 surface, glossy and of a whitish colour ; they rise from a small 

 circular disk composed of aborted cells, narrow and stem-like 

 for a short distance above it and then widening out (PI. XIV. 

 fig. 6 c). P. Skenei, so far as I have seen, originates in an 

 irregularly spreading crust, largely composed of normal cells, 

 and its surface bristles with tall mucronate processes. The 

 latter are totally wanting in the variety, and with them the 

 most characteristic feature of the normal form disappears. 

 The central avicularium is placed within the margin of the 

 peristome and is barely visible. In P. Skenei, n., the whole 



* Every element of structure is on a larger scale in the present form 

 than in tiie normal P. Skenei. 



