286 Mr. Toulmin Smith on the Classification 



places. I am unable therefore to describe the outer plaits. The 

 characters of the inner ones are, however, so marked that those 

 of the outer ones are quite unnecessary in order to establish the 

 specific difference. 



The name retrusus may be considered either to express the ex- 

 traordinary degree in which the inner plaits are drawn back to 

 form the projections ; or that the most marked characters of the 

 species are hidden from external observation by being on the in- 

 ner plaits. In either sense the name seems equally appropriate. 



7. Cephalites catenifer. PI. XIV. figs. 9, 14, 15, 16. 



Plaits broad and deep : outer plaits projecting prominently at 

 irregular intervals; projections horse-shoe shaped, with one 

 arm of a lower projection often linked to the hoop of the pro- 

 jection above it on the same plait ; occasional points of ana- 

 stomosis between adjoining plaits : inner plaits having large 

 and generally oval depressions at regular intervals ; bulging 

 on each side around depressions till adjoining plaits meet and 

 open into each other : processes very conspicuous : wall very 

 thick. 



Var. Annulatus. 



Plaits broad and deep : outer plaits projecting prominently at irre- 

 gular intervals; projections ring-shaped, and generally running 

 into each other on the same plait and often anastomosing with 

 those on adjoining plaits so as to form connected rings over the 

 whole surface : inner plaits having large and generally oval 

 depressions at regular intervals ; bulging on each side around 

 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 PI. XIV., all taken from the same spe- 

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

 tion of the fold. Fig. 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 



