50 PROCEEDINGS OF THE NATIONAL MUSEUM. 



almost wholly upon minor modifications of the opercular characters, 

 and, in some instances, upon certain distinguishing shell characters 

 but only where such consideration does no violence to the more 

 important opercular evidences. 



To further refine the classification we have created a generous num- 

 ber of subgenera based almost wholly upon shell characters, using 

 especially the "breathing apparatus" and the sculptural features in 

 their various combinations. A final division under the subgenera 

 carrying the classification to its ultimate rational end has been 

 adopted by designating specific groupings wherever obvious similarity 

 in form has made such assemblages useful for systematic study. 



Among the numerous shell characters employed in om- classification 

 we have considered the most important the presence or absence of 

 punctures or slits within or on the edge of the aperture, and when 

 present, the nature of such devices for enabling the animal to obtain 

 air when the operculum is withdrawn or seals the aperture. This 

 character we believe to be of less taxonomic value than that of the 

 opercula but of paramount importance among the shell characters. 



Among the Annul ariidae we have found no important radular 

 differences and no range in nuclear characters worthy of note. A 

 sufficiently large number of species from the different genera have 

 already been described anatomically, or observed by ourselves, to 

 warrant our belief that no fundamental anatomical differences exist 

 within the scope of the entu-o family. The length of the proboscis 

 seems to be merely relative. The division of the foot by a longitudi- 

 nal groove into two independently functioning muscular masses is 

 always present, but the peculiar method of progression caused thereby 

 is merely exaggerated in those species where the foot is short and less 

 apparent when the foot is longer. 



OPERCULUM. 



The outer periphery of the opercular whorls usually tapers to a 

 very thin edge, which becomes upturned as the operculum is drawn 

 into the aperture to close the shell. This upturning of the free edge 

 is the introduction of the lamellar formation- which we find so won- 

 dei-fully diversified in this family. Even in Chondropoma, which was 

 originall}^ defined as a simple chondroid plate, we find that these 

 outer uptm-ned edges are not always cemented down flat upon the 

 upper sm-face of the succeeding turn, but at times are left as a sug- 

 gestion of a slender lamella, though more often they are worn away, 

 leaving only an indication of a sutural thread. From this we can 

 easily develop the Adamsiellinae, in which the inner edge of the 

 whorl backs up and strengthens the slight chondroid upturned outer 

 edge of the preceding turn until a strong, simple, elevated, calcified 

 lamella is formed. In the Annularinae this process is earned a step 



