466 PELORID A. 
by placing the strong corneous operculum well within the 
margin; it has nothing of the rapid retraction of the foot, 
as in Rissoa. 
These animals abound of large size in the Warren Sands, 
opposite Exmouth. When just taken, in vigour, and im- 
mersed in sea-water, it is scarcely possible to contemplate a 
more beautiful and interesting object, with its shell rising as 
a globular pyramid from an immense circular disk, elegantly 
marked with fine dark lines on a clear drab ground. I re- 
commend this species for examination ; its large size affords a 
good view of the external organs, and the anatomy is compa- 
ratively facile. 
N. niripa, Donovan. 
N. nitida, Brit. Moll. i. p. 330, pl. 100. f. 2, 3,4; (animal) pl. P.P. 
f. 5 (as Alderi). 
N. Alderi, nonnull. 
This species, as regards the external organs, is so similar to 
the N. monilifera, that a notice of the variations will suffice. 
It has, like its congener, two branchial plumes, and the 
mucous fillets are nearly as evident as in the Muricide. In 
the male, the organ of reproduction is in every respect more 
developed. The eyes are distinctly visible, immersed in the 
centre of the anterior bases of the tentacula. The colour of 
the upper front surface of the foot and tentacula is a deepish 
dull red-brown, which is deposited on the first-named organ 
in close, irregular, longitudinal streaks, but on the posterior 
portion they are more distant and paler coloured, and still 
more so laterally ; the sole is a uniform pale yellow. These 
animals vary much in the general colour, but whatever the 
ground colour may be, it is modified on the particular parts 
as above stated. | 
The animal is active and not uncommon in the coralline 
zone at Exmouth, where, though very rarely, the pure snow- 
white variety occurs, as well as the plain chestnut-coloured 
N. sordida. 
