410 PROCEEDINGS OF THE ACADEMY OF [1896. 



atose condition •which follows is only broken up bv the presence of 

 moisture in the air, which the prisoner perceives and takes advant- 

 age of to return to active life. The state of torpor may occasionally 

 last for years, but is general among land shells during the dry sea- 

 son in the tropics and during the winter of the colder zones. Most 

 of the collections made at the Galapagos seem to have been made in 

 the dry season. This was the case with Darwin's work and all the 

 Bulimuli collected in a living state by Dr. Baur retain the whole or 

 portions of the epiphragm, showing that they were in retirement 

 when taken from the trees. If the creature, by an early diminution 

 of humidity, is forced into its state of hibernation before its normal 

 period of growth is absolutely completed, it frequently happens that 

 the portion of the shell about the aperture is irregular and bears 

 indications of having been secreted under abnormal conditions. The 

 incremental rugse in the vicinity of the margin will be exaggerated 

 or crowded, the color of this part of the shell absent or different 

 from the rest, the pillar irregularly tuberculose or keeled at the 

 base ; abnormal thickenings or tubercles may appear on the outer 

 lip or on the parietal portion of the aperture, and the margin of the 

 lip will take on an irregular form, presumably to adapt itself to the 

 irregularities of the surface to which the creature is about to attach 

 itself for hibernation. Reeve's figure of Bulimulus Danvini shows 

 a state of affairs such as I have described, so does the form figrured 

 under the name of ^. Simrothi by Reibisch, and similar indications 

 are afforded by specimens of 5. nux, B. rugulosus,B. tortuganus and 

 B. Bauri. An understanding of these facts is necessary in order to 

 avoid the use of these temporary and individual dynamic mutations 

 as specific characters, an error several authors have not succeeded 

 in escaping. 



To return to the modification of the surface of the shell by local 

 conditions, the facts above cited enable us to understand how under 

 normally favorable conditions the organism deposits the mucus mat- 

 ter, which, by a process analogous to the crystallization of salts in a 

 colloid medium, hardens into the shell substance, which then forms 

 a compound of crystallized lime (aragonite) and conchioline (not 

 chitine as stated by Osborn* and others). 



2s ow if we assume the attenuated film of secretive tissue constitu- 

 ted by the margin of the mantle expanded, in order to divest itself 



* Studies from the Biol. Laboratory, Johns Hopkins University, II, p. 431, 

 1883. 



