24 University of Michigan 



and it snaps quickly into its shell when disturbed. It occasion- 

 ally suspends itself from leaves, by a mucous thread 2 or 3 cen- 

 timeters in length. 



The operculum of T. plicatula has been described as like 

 that of Chondropoma (Cf. Henderson and Bartsch, 1920), 

 but fresh specimens have a well-developed, calcareous portion, 

 parallel to the "chondroid" plate, and the outer, calcareous 

 plate is supported by fine, vertical lamellae parallel to the 

 growth lines. Each whorl of the calcareous portion is not 

 quite so wide as the underlying whorl of the horny, basal plate. 

 On account of its thin, fragile nature, the calcareous plate is 

 quite easily crushed and brushed away, so that only the chon- 

 droid portion is left. Its fragility may be correlated, in part, 

 with the acidity of the leaf-mould, and the absence of lime- 

 stone in the region studied. Structurally, this operculum does 

 not differ greatly from that of T. williamsoni (see below), but 

 its whorls increase more rapidly in size, so that the nucleus 

 is more eccentric. (See fig. ii-9.) 



The shells are very variable in color and texture. Usually, 

 spiral bands (as many as 10), of chestnut brown to purplish, 

 are present, but these are almost always broken into dots. The 

 dots tend to be arranged in vertical rows and, in a few speci- 

 mens, these are joined together by dark varices, so that the 

 bands become vertical instead of spiral. The ground-color 

 varies from light cream to dark fulvous. Most of the speci- 

 mens from the edge of the hills (stations 22, 28) are dark 

 colored, and are ver>- commonly quite unicolor. 



The sculpture varies from rounded ridges parallel to the 

 growth lines, to marked, thread-like riblets. Usually some 

 buttress-like denticulations are present in the sutures, but their 

 numl)er and size is very variable. The strength of the umbil- 



