VII. Mastigocriirus loreus. 199 



tlnown ii)to strong lateral folds. I'rlinitively the ossicles are 

 hexagonal in outline and alternate with those of adjacent 

 rows, but the simple outline is usually obscured by the 

 folding. 1'he 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 

 })lates, in which the folds of t!ic 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 ajjpearance 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 A 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 fllled with matrix that both 

 Mr. Hollick and Mr. Wilson, experienced scientific artists, 

 drew the plates as though they were quite discontinuous, and 

 as though tliere 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. 



