282 MTRIOZOID^E. 



and by no means the most marked, under the name of 

 Hippothoa porosa. 



The cells in our British specimens usually present a 

 very flat surface, and are rhomboid in figure ; the walls 

 are of dense texture ; and the border is perforated by a 

 single row of pores. In some cases, however, the shape 

 varies to ovate, the surface is convex, and the marginal 

 perforations are wanting. The vibraculum also varies 

 much in length. In a form from the Red Sea (?) the 

 species wears a very different aspect. The cells are nearly 

 half as large again as in British specimens, ovate and very 

 convex, and a good deal narrowed above; the thickened 

 peristome surrounds the whole orifice, instead of being 

 confined to the upper part of it; the entire surface is 

 covered with coarse punctures ; and the vibracula, which 

 are slender and whip -like, attain an enormous develop- 

 ment. The vibracular cell is also placed much nearer to 

 the orifice than is usual. This is probably the porosa of 

 Smitt (Plate XXXVII. fig. 5). In a yet more remarkable 

 form from South Africa the shape of the orifice has under- 

 gone modification, and the central sinus has disappeared ; 

 the ovate cells, which do not exceed in size those of our 

 British form, are covered with puncta ; and a little below 

 the inferior margin of the mouth there is a small circular 

 pore. Perhaps the most remarkable change has taken 

 place in the vibracula, which, instead of being slender and 

 thread-like, present a rigid appearance, are of moderate 

 length, broad below, and tapering gradually to a point, 

 and bear no slight resemblance to a miniature sword or 

 scimitar. This very marked form is worthy of being 

 recorded as M. Hyndmanni, var. ensiformis, if it is not 

 entitled to specific rank (Plate XXXVII. fig. 6). 



HABITAT. On stones, shells, &c., from deep water. 



LOCALITY. Sana Island, off the south-east of the Mull 



