142 UNIVALVES__. TROCHUS. ~ 
ture of the shell in this as well as in many other genera of 
univalves, is closed by a stony or horn-like operculum, 
affixed to the animal. 
Of the divisions of the Trochus the first is the most 
numerous; it comprehends those species which have 
their pillar perforated, and is divided into five fami- 
lies, distinguished by the peculiarities of the pillar 
and umbilicus: the T. niloticus, T. maculatus, T. pha- 
raonis, T. cylindraceus, and T. perspectivus, may be 
mentioned as illustrative examples of each family. 
The T. conchyliophorus, or Carrier Trochus, is a very 
remarkable species; it is invariably covered with ex- 
traneous substances, strongly adhering to the whorls 
of the shell. There are two distinct varieties: one of 
which is familiarly called the Conchologist, from its 
being loaded with perfect shells or fragments; and the 
second, the Mineralogist, as its adhesions consist of 
stones, ores, &c. When the former variety is loaded 
with corals only, it is called the Zoologist or Coral= 
carrier. 
The second division consists of those Trochi which 
are imperforated, or have their umbilicus closed, as in 
the T. tuber, (which greatly resembles a Turbo) T. labio, 
and T. vestiarius. The T. iris of this division deserves 
particular notice, on account of the iridescence and 
splendid metallic lustre which its surface exhibits when 
uncoated. 
The third division includes those species which are 
much elongated, and greatly resemble screw or needle 
shells. Unlike the rest of the genus, they have an ex- 
serted pillar; and, when placed on their base, they fall 
