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cies, the central opening is in a smooth concave surface, which is sur- 
rounded by a finely crenated narrow margin, as is shewn in Fig. 21. 
In the third species, the opening is in the centre of a circular, de- 
pressed, central surface, which is surrounded by a broad, raised, and 
finely crenated margin, as delineated in Fig. 22. No representations 
are given of the entrochi which are formed by these three species of 
trochitffi, since, as their margins are similar, no difference can be ob- 
servable in their articulations, when viewed externally : for similar 
reasons the figures of several of the entrochi are omitted. 
The fourth species. Fig. 23, is of a very curious structure. Its five- 
rayed opening is surrounded by a small finely striated circular area, 
around which is a smooth circular groove, surrounded by a raised 
marginal ridge, the upper and inner part of which is finely crenated. 
The corresponding trochites with the last appears to be the one re- 
presented Fig. 25, the body of which bulges out considerably, and 
its articulating surface, which is formed of a smooth area round 
the central opening, surrounded by a margin crenated on its outer 
part, is exactly fitted to be received into that of the former trochites, 
and to constitute a series, such as is represented Fig. 44. 
The most common form of the stellated opening is that with five 
rays. There are deviations, however, from this form, as in the tro- 
chites Fig. 26, where the central foramen has three obtuse rays, from 
which proceed minute striae, which pass directly to the margin. 
By different arrangements of trochitae, varying like those repre- 
sented Fig. 23 and 25, in their size and form, entrochi of different 
figures are obtained, as are instanced in Figures 5, 6, 8, 42, 43, and 44. 
The specimen Fig. 8, which is a little bent towards its upper part, 
is particularly interesting, since it serves to demonstrate that this 
peculiarity of form is particularly fitted for those entrochi which 
form that part of the trunk of the animal which is nearest to its head. 
But this circumstance will be particularly attended to, when inquiring 
into the uses of these parts. 
