THE MOUNT DESERT REGION 339 



in size, hemispherical, without pores of any kind; instead, it 

 is finely granular over the whole of the exposed surface at 

 all stages ; it becomes thick-walled in older stages of calcifica- 

 tion, but it is never covered and obscured by encroachment 

 on the sides by neighboring zooecia. The frontal surface of 

 the zooecium also becomes very thick with the early forma- 

 tion of the tremocyst, but presents much the same appearance 

 as in younger stages. A delicate granular surface is pre- 

 sented in final calcification, as in hella. Also a rounded or 

 short-pointed umbonate process is sometimes present proxi- 

 mal to the secondary aperture. 



The zooecia measure somewhat smaller than in bella, about 

 0.6 mm. in length by 0.4 mm. in breadth, while the primary 

 aperture is a trifle larger, about 0.14 mm. long by 0.16 mm. 

 wide. 



Probably another genus will be required for these and other 

 thick-walled porous species, since they seem to differ materi- 

 ally from typical SmitUna, but for the present I prefer to 

 assign them to this genus. 



MucRONELLA Hincks, 1880 



MucRONELLA iMMERSA (Fleming), 1847. PL 11, fig. 8; pi. 15, 

 fig. 9 a. (Osburn, 1912, p. 243 (.¥. peacliii), for references and 

 synonymy; Whiteaves, 1901, p. 107 {M. peachii), Canadian 

 records.) Common on stones at both shore and dredging 

 stations; taken at eight shore and thirty-three dredging sta- 

 tions. An abundant North Atlantic species from Spitzbergen 

 and Greenland southward to the British Islands and southern 

 New England. The primary characters include a row of mar- 

 ginal pores, a rounded aperture with a conspicuous tooth on 

 the proximal border, and 5 or 6 slender spines on the oral 

 margin. The ooecium is hemispherical and imperforate. 

 Secondary calcification is often heavy and may obscure most 

 of the primary characters. The smooth front of the zooecium 

 then becomes rough, and grooves may extend inward from 

 the marginal pores. 



