VII. Mastigocrinus loreus. 199 
thrown into strong lateral folds. Primitively the ossicles are 
hexagonal in outline and alternate with those of adjacent 
rows, but the simple outline is usually obscured by the 
folding. The folds on one side of one ossicle usually meet 
the folds on the adjacent side of two adjoining ossicles. 
Sometimes, however, there appear to be small intervening 
plates, in w vhich the folds of the ossicles on either side meet. 
The stereom is perfectly continuous over the whole surface of 
the ventral sac, forming the floor of the depressions just as 
much as the summits of the folds and ridges (zincotype, 
figs. 6 and 7). That the sac was a very flexible structure is 
proved by the appearance presented by the upper and lower 
sutural surfaces of the individual ossicles: when the ossicles 
are slightly pushed over so as to expose these edges, it is 
seen that the end of each ridge is a regular articular facet of 
almost circular outline, marked with depressions for ligament, 
which depressions run at right angles to the surface of 
the sac (zincotype, fig. 8). The plates that form ridges 2, 3,, 
and 4 are more or less flat; but those forming ridges 1 and 
5, which are at the borders of the sac, are bent round at 
a sharp angle, the angle itself being rounded and constituting 
the ridge. In other words, these two ridges form part of both 
the anterior and the posterior surfaces of the sac. The 
following are measurements of ossicles from the proximal 
region of the sac :—In the median ridge (3), height ‘83 millim., 
width 3 millim.; in ridges 2 and 4, height ‘7 millim., width 
2-6 millim. ‘The ossicles decrease in all dimensions in the 
distal region of the sac. 
The description of the Ventral Sac has been given at con- 
siderable length, as the specimen 57048 B.M. shows the 
structure of this interesting organ more clearly than any other 
fossil known to me. It is perfectly certain that there are no 
slits or pores of any kind in this sac; and yet so deceptive is 
the appearance of the depressions filled with matrix that both 
Mr. Hollick and Mr. Wilson, experienced scientific artists, 
drew the plates as though they were quite discontinuous, and 
as though there were large slits between the folds or finger- 
like processes. Now, however, Mr. Hollick, after examining 
the whole surface of the sac with a microscope, and after 
seeing the matrix cleaned out with needle and brush from some 
of the most slit-like depressions, is so convinced of his former 
error that he will hardly permit me to publish the drawing 
that he first made. This, however, I give in the margin, in 
order to show how the most careful observer may be misled. 
These facts seem sufficient explanation of those statements, 
so often controverted in the present series of papers, as to the 
presence of slits in the ventral sac of the Fistulate Crinoids. 
