326 VEKMETIDvE. 



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 in 

 Caecum, though the foot is regular, it is singularly short both 

 before and behind, and, as in Vermetug, 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 Siliquaria. This remark is made to show that 

 the operculum of Caecum glabrum, our proposed Vermetidan, 

 exhibits the precise features of the same organ in Siliquaria, 

 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 foraminiferous Polystomellce. 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. Philippics description of Siliquaria, which, if it had been 

 written expressly for our C. glabrum, could not be more 

 apposite : 



" Operculum valde mirabile, legumini medicaginum simile, 

 cylindricum, uti trochlea e lamella spirali quinquies circa 

 axim revoluta constans. Hsec lamella cornea, subtus niti- 

 dissima, supra farinacea 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. 



