326 VERMETID Z. 
assuming the aspect of a cylindrical plug, with a circular 
operculum at its extremity. This foot, from the shape and 
probable fixity of the shell, cannot serve for reptation ; but im 
Cecum, though the foot is regular, it is smgularly short both 
before and behind, and, as in Vermetus, carries a circular 
operculum. Notwithstanding these variations, both animals 
are simple holostomatous mollusca. These differences have 
been dwelt on, to sanction a generic separation between the 
two; but we repeat, that their organs, though modified, are 
so essentially the same, as to create doubt if these variations 
are more than mere phases of development, arising from the 
effects of fixation and locality, and in no way invalidate the 
present arrangement, which we will confirm by additional 
facts. And first we observe, that Vermetus differs very 
slightly from Siliqguaria. This remark is made to show that 
the operculum of Cecum glabrum, our proposed Vermetidan, 
exhibits the precise features of the same organ in Stliquaria, 
and, like it, is rolled as round a windlass in 5—7 coils, that are 
crenated at the edges, mammillated above and below, and on 
the summit has numerous loculi, like those in the centres of 
the foramiiferous Polystomelle. This curious fact exhibits 
one of the strongest links of affinity between the three genera. 
That malacologists may compare the two opercula, we extract 
M. Philippi’s description of Stliqguaria, which, if it had been 
written expressly for our C. glabrum, could not be more 
apposite :— 
“ Operculum valde mirabile, legumini medicaginum simile, 
eylindricum, uti trochlea e lamella spirali quinquies circa 
axim revoluta constans. Hee lamella cornea, subtus niti- 
dissima, supra farimacea subpubescens, margine subtus striolis 
brevibus subcrenata. Centrum subtus et supra convexum, 
papillam centralem et loculos plures ostendens, et exactissime 
Cristellariam s. Robulmam aliquam referens.” 
The operculum of C. glabrum agreeing closely with this 
description,—and it is in this case a most important coinci- 
dence, as these are the only two molluscan genera which have 
so curious an opercular mechanism,—seems to stamp with 
certainty the alliance of the three genera. 
