79 



depressions till adjoining plaits meet and open into each other : 

 processes very conspicuous : wall very thick. 



This is a singular species. The specific name of the typical 

 specimens exactly expresses the appearance of the outer surface, 

 which looks as if several links of a chain were hung about it, 

 sometimes disconnected, often connected, always, or almost 

 always, open on one (and generally the same) side. 



This species will be readily distinguished from C. bullatus by 

 the fact that the semilunar fold is continued down to the upper 

 edge of the plait, as well as by the links being so often con- 

 tinuous, and by each individual projection being much larger. 



Figs. 14, 15 and 16 of PL XIV., all taken from the same spe- 

 cimen, will probably assist in the understanding of this modifica- 

 tion of the fold. Pig. 14 shows a part of the core of the matrix, 

 that which filled the central cavity. The round spots are where de- 

 pressions existed in the body itself, and where, consequently, the 

 matrix projected outwards from the core. Being broken off at 

 each place, these regular marks are left, contrasting strongly with 

 the portions of the membrane adhering to the matrix elsewhere*. 

 Fig. 16, which should be compared with fig. 14 of PI. XIII., 

 shows the peculiar elevations on the plaits : and fig. 15 is a trans- 

 verse section showing three plaits ; the uppermost being struck 

 at a point where there is not any projection, the two others just 

 at the bend of two projections. 



The specimens which I have distinguished as a variety, under 

 the name of annulatus, appear to be cases in which the horse-shoe 

 elevations have become more than usually continuous both on the 

 same plait and by anastomosis with those on adjoining plaits. 

 This character is sometimes seen on the lower part of specimens 

 the upper part of which exhibits the true normal characters of 

 C. catenifer, as in fig. 9. PL XIV. In some cases, however, the 

 same appearance of connected rings, instead of rows of open 

 links, covers a large part, or the whole, of the surface ; and it is 

 important that the true place of such specimens should be un- 

 derstood, whence the utility of distinguishing them as a variety of 

 C. catenifer. 



It generally happens that, even in the most characteristic spe- 

 cimens of this variety, there are places in which the projection 

 on the outer plait stands, as it so often does in the normal C. ca- 

 tenifer, single and wholly unconnected with any other projection 

 on the same or on any adjoining plait. In that case, instead of 

 being horse-shoe shaped, the circle is usually complete. We thus 

 find, on an external plait, a fold very similar to that which cha- 

 racterizes the inner plaits of C. retrusus. 



* See ante, p. 60, note f. 



