﻿76 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



As is shown on the graph by the open circles, which are plots of shell 

 width against angle of spire, there is no correlation between the size, 

 hence age, of individuals and the angle of the spire. Not only is there a 

 marked variation in spire angles but there appears to be a minor, 

 random fluctuation of this angle in the same individual during its 

 life. When this occurs, the sides of the spire are not flat but are either 

 slightly convex or concave. The mean angle of spire for 23 specimens 

 was 31.93°. However, measurements of spire angles are difficult to 

 make, and on the whole are not altogether reliable. 



The shells of the Lithia Spring populations closely resemble those 

 of Guam Island, except in being considerably smaller (see under 

 bionomics, p. 104). The red spottings of the Lithia Spring popula- 

 tions are more pronounced and tend to amalgamate axially into small 

 flammules. In larger specimens, however, the coloring is identical 

 with that found in Guam individuals. 



Operculum (fig. 33, D) : The operculum is corneous, opaque, dark 

 reddish brown, paucispiral with the nucleus placed in the far bottom 

 right corner. In very old specimens the nucleus is sometimes chipped 

 or worn off the operculum. Growth lines coarse and irregular. The 

 inner side of the operculum glossy and smooth with a large elongate, 

 depressed scar from the muscle of attaclnnent. In very small speci- 

 mens the operculum is transparent amber colored, with the nucleus less 

 excentrically placed and usually less elongate in shape. In younger 

 specimens the operculum is approximately two-thirds as wide as it 

 is long, while in old individuals it is half as wide as long. 



Foot (fig. 33, A, C) : The foot is proportionately small in comparison 

 with that of most gastropods. The flat, creeping surface is squarish, 

 straight edged at its anterior end and slightly truncate posteriorly. 

 The operculum is attached to the dorsal surface at the posterior end. 

 Above this, and continuous with the general musculature of the foot, 

 is the heavy, curved columellar muscle, which is attached to the inner 

 columella of the shell. The anterior, leading edge of the foot bears 

 across its entire width a narrow mucus slit. From this exudes a 

 mucus, which aids the foot in sliding over sand or mud. Immediately 

 above and continuous with the thick foot are the head and proboscis. 



Head (fig. 33, A) : The head is ill defined but may be said to include 

 that region posterior to the proboscis that bears the two tentacles, and 

 it contains the brain or central ganglia. Immediately posterior to it, 

 in mature and gravid individuals, lies the brood pouch for the ovovivi- 

 parous young. The tentacles are a little longer than the proboscis, 

 round, filiform, and very slightly swollen at the base, where the small 

 black eye is located. The tentacles are capable of being swung in an 

 arc of about 45° in any direction but are rarely bent, curved, or coiled 

 to any great degree. Upon stimulation they may be shrunk to half 



