MUCRONELLA PAVONELLA. 377 



GEOGRAPHICAL DISTRIBUTION. Gulf of St. Lawrence 

 (Dawson) : Greenland, off Frederickshaab, 100 fathoms 

 (Wallich) : Nova Zembla, Jugor-scharr, 10-14 fathoms 

 (Stuxberg and Theel) : Spitzbergen, 20-60 fathoms (To- 

 rell and Swed. Exped.) : Finmark, 20 fathoms (Goes and 

 Malmgren) : west off Jutland, 26 fathoms (Kirchenpauer) . 

 Not yet found in Southern Scandinavia (Smitt). 



This is a remarkably beautiful species. In its incrust- 

 ing state it forms large patches (an inch in diameter) 

 perfectly circular in shape, slightly depressed in the centre, 

 and wearing the appearance, even to the naked eye, of 

 most delicate lacework. When growing erect it assumes, 

 according to Mr. Alder, the form Hemeschara or Eschara 

 (of authors), according to the substance on which it is 

 developed. 



The zocecia are large and regularly arranged in quin- 

 cunx; the surface is ornamented with ribs, which pass 

 off from the margin arid radiate towards the centre of the 

 cell, inclosing rather large loop-like areolar spaces. In 

 the older portions of the colony these are sometimes 

 almost obliterated ; but they are generally a conspicuous 

 and characteristic feature. Smitt states that the mandible 

 of the avicularia, though occasionally round, is usually 

 triangular. This is certainly not the case in any of the 

 specimens which I have examined, whether from this 

 country or from Greenland and North America. In all 

 cases I have found the rounded mandible only. 



The mucrouate elevation of the peristome, which is the 

 usual characteristic of the present genus, is wanting in 

 M. pavonella; but its affinities, on the whole, are with 

 this group. 



