1913.] NATURAL SCIENCES OF PHILADELPHIA. 9 



mark the swelling of the lirae that is the cause of the lighter spots, 

 they run from 16 to 20 to the whorl, and when this periodicity is 

 seen with the whitish spots reaching the periphery, this in turn 

 becomes tuberculated and keeled. This regularity of the markings 

 is characteristic of the younger stages, and as long as it is seen the 

 young shell will be found to be keeled and tuberculated; when it 

 disappears and the color pattern becomes a mottling of the shell, then 

 the periphery is neither keeled nor tuberculated. In the two places, 

 where dwarfed shells were taken, a large number of them have 

 this tuberculated and keeled periphery on the last whorl, although 

 considerably less than half are so ornamented. It is noticeable 

 that the whitish ornamentation, when it is accompanied by the pe- 

 ripheral tuberculation, follows the growth lines, and is then more 

 continuous and stronger. But when it curves towards the mouth 

 of the shell and crosses the growth lines, it tends to break up into 

 branches, rarely reaches the periphery, and practically never causes 

 tuberculation. .When the animals are living under optimum condi- 

 tions, as at the Somerset and Somerset Road stations, this bending 

 forward of the ornamentation across the growth lines becomes 

 characteristic, and the color pattern becomes finer and less regular 

 by the branching of these whitish lines or by their breaking up into 

 dots. And where this finely mottled color pattern is seen, the last 

 whorl is nearly smooth and the periphery is free from tubercles. 



The tuberculated condition of the periphery is thus a character 

 of the young stages, sometimes continued into the adult stages 

 up to the development of the lip. The appearance of this character 

 in the dwarfed races (Plate I, figs. 11-15) at Kendal Road station 

 and at the Sturridge station is due to what is generally described 

 as ''reversion to an ancestral form"; or it is due to the animals, 

 living under unfavorable conditions as regards shell development 

 and growth, which causes them to mature at an earlier stage than 

 those forms which live under optimum conditions. But while the 

 conditions are unfavorable to growth, they are not unfavorable to 

 reproduction, for these dwarfed forms are very plentiful at the 

 localities where they were taken. An examination of the shells 

 of these Kendal Road and Sturridge forms will show at once from 

 the growi:h lines that they grew with many interruptions, as many 

 as 30 such interruptions being often seen in one shell. Those from 

 Kendal Road station also mature earlier, as they have only five 

 whorls, instead of five and one-half or six, as in forms growing under 

 more favorable conditions. The forms that live in walls, whether 



