266 PKOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



beads oii a striug, all formiiig part of one necklace. WUeu, by study of 

 the conditions with which we tiud certain characters associated, we 

 shall come to understand why a certain shell in one place is coarsely 

 striated, in another nearly smooth; here cariuate and there rounded; 

 here elevated and there depressed ; thvn we shall begin to grasp the 

 thread connecting the whole scries and to have a scientific basis for our 

 systematic arrangement which will replace that which is now disinte- 

 grating. That we t-hall reach such a basis there are strong reasons to 

 hope, and with less delay than would at first sight seem j)ossible. 



The grouj) of related forms for which eereulus Muhlfeld is the oldest 

 name, exeuiplities the law that where anj^ character or organ exists in 

 unusual duplication it is more liable to variation than where it is less 

 numerously developed. The number of whorls is unusually large and 

 also quite inconstant. The spire may be moderately elevated or even 

 concave ; the base may exhibit more or fewer whorls ; the central um- 

 bilical pit be more or less fiiniculate ; the stritie above weaker or stronger; 

 the periphery nearly circular or quite gibbous ; the vertical diameter 

 may vary 100 per cent. ; the parietal lamina (as pointed out by Tryon) 

 may be present or absent, and the angle of the aperture with the axis 

 varies widely. 



The shell generally has from five to seven well marked whorls ; but, 

 under especially favorable conditions, as near the walls of Fort Marion, 

 at Saint Augustine, it may have ten or twelve. The number exhibited 

 on the base isr absolutely inconstant when specimens from various lo- 

 calities are compared. Only by breaking into the last whorl can 

 it be determined whether the parietal lamina exists. In specimens la- 

 belled scptemvolva by Mr. Binney I have found it, and it is often absent 

 in what is otherwise typical cereohis. The soft parts in all are very 

 uniform. The chief forms may be distinguished as follows: 



1. H. ccreolns septcmvolva. 



2. H. cereolus Jaminifera. 



3. S. cereolus microdonta. 



4. H. cereolus cereolus. 



No. 1 is extremely depressed, thin, flat, with a carina on the upper 

 edge of the last whorl; moderately strong sculj)ture above and the reflec- 

 tion of the tip narrow. Lamina usually absent. This is what Mr. Bin- 

 ney, in naming the Smithsonian shells, has considered to be the typical 

 septemvolva of Say, though Say's diagnosis would not distinguish his 

 shell from other varieties except the next one. It is more restricted in 

 range than most of the varieties and has been reported chiefly from 

 Florida. It does not exceed 10'""' in maximum breadth, except in very 

 unusual instances, and generally averages less. 



The large form figured and described by Binney (Terr. Airbr. Moll., p. 

 281) is not that described by Say in his original diagnosis, though the 

 shell described by Say might perhaps have been closely related to it. 

 His types were 0.3 and 0.4 inch in diameter, or lO"'"' to the IS"""^' form 

 figured by Binney. 



