MICROPORELLA VIOLACEA. 217 



tubular pores scattered over the zoarium, generally one 



or two above the orifice. 

 Var. /S (plagiopora) . The central avicularium very large, 



and turned obliquely to one side. 

 Colonies forming large, rather thick crusts, bright and 



glossy when fresh, spreading irregularly, and of a dark 



purplish or creain colour. 

 Yur. a seems to be always of a light colour. 



K OF VARIATION. The cells vary slightly in shape, 

 being sometimes ovate or subpyriform, and sometimes 

 hexagonal or lozenge-shaped. They vary more remark- 

 ably in size ; and within the limits of one and the same 

 colony cells of the most different dimensions are found 

 associated (Plate XXX. fig. 4). They are frequently 

 punctured round the margin, and the punctures are some- 

 times channelled. In young cells a number of narrow 

 loop-like fissures or areolae extend from the margin to 

 the middle. These disappear with the progress of the 

 calcification, which materially changes the appearance, 

 and destroys much of the beauty of the species. In 

 some cases the punctures extend over a large portion 

 of the surface. In certain states the walls are much 

 thickened, and covered with large perforations, and pre- 

 sent a coarsely reticulate appearance. In old specimens 

 the cells are almost confluent, the zoarium is of a dead 

 white colour, flattish, of granular texture, and sometimes 

 thickly perforated. The avicularium is only represented 

 by a foramen a little below the orifice. 



In the curious variety a, the elevation of the cell towards 

 the oral extremity is a very marked peculiarity ; the 

 avicularium is much more elongate than in the normal 

 form, with a very slender mandible ; the one central pore 

 is replaced by a pair ; and a new feature makes its appear- 

 ance in the raised tubules (? aviculiferous) which are dis- 



