PORELLA STRUMA. 329 



b. Zoarium incrusting, or erect and unilamellate. 



PoRELLA STRUMAj Norman. 

 Plate XXXIX. figs. 3-5. 



Hemeschara struma, Norman, Quart. Journ. Micr. Sc. (n. s.) viii. 221, 

 pi. vii. figs. (i-8. 



Zocecia obovate, slightly convex^ quincuncially arranged, 

 the cell-wall much swollen immediately below the mouth, 

 surface smooth or roughened by anastomosing ridges, a 

 line of wedge-shaped areolae round the margin, which 

 become much more distinct with age ; orifice (adult) 

 broader than long, upper margin arched, the sides 

 straight and slanting inwards, lower margin somewhat 

 curved outwards ; peristome unarmed ; a rounded avicu- 

 larium within the lower lip. Ocecia rounded, not 

 much raised, smooth and silvery, or slightly roughened ; 

 closely united to the cell above. 



Colonies forming a yellowish, glistening crust, which rises 

 here ' and there into free frill-like expansions, not 

 exceeding half an inch in height, and consisting of a 

 single series of cells. 



Range op Variation. The description is founded upon 

 comparatively young specimens. With age the species 

 takes on a very different aspect. The front of the cells 

 is covered with a thick glassy crust, which completely 

 conceals the characteristic goitre-like enlargement below 

 the mouth, obliterates the sutures between the cells, and 

 forms a perfectly flat surface. On this the marginal 

 areolae appear as deep fissures^ while the central portion 

 is filled up with anastomosing ridges, separated by cavi- 

 ties of various size and shape. To add to the disguise, 

 the cells are now inclosed bv thick raised lines, which 

 mark them out with great distinctness. In this condition 



