62 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xii. 



In group 13, weU defined undulations are even present at the class 1 stage, plate 41, figure 9, 

 and are frequent at the class 2 stage, and are the prevailing type in class 3. Some individuals 

 remain roughly and irregularly corrugated, but essentially a smooth sheU up to the class 3 stage, 

 and then become nodulose, as in plate 41, figure 10. In some cases the nodules are present up 

 to the class 3 stage, and then are possibly less developed or practically absent at maturity, as 

 in figiu'es 2, 8, and 10. Numerous individuals in lot 94 show aU degrees from normal develop- 

 ment to almost perfect inversion. This is shown on the body whorl by the presence of lower 

 nodules, a smaller number on a whorl, so that only a part of a whorl possesses them. There are 

 only a few of the relatively smooth shells in this series, and yet the different degrees of inver- 

 sion are abundant. In lot 157, from Fishdam, Tenn., there are also inverted individuals. Com- 

 pared with group 12 the nodulations of group 13 is more uniform and it generally develops 

 at an ear her age. 



In gi'oup 14, inverse development of spines was observed in lot 112, in which two individuals 

 were found which were nodulose or spinose at the class 3 stage, and became less so or smooth 

 on the last whorl. Similar examples are also found in lot 175 (Nos. 3, 27, and 79). Inversion is 

 shown on plate 42, figures 16, 21, and 29. Several of such shells are found in lot 178, in a very 

 spinose population. 



In group 15, from Chissolms Ford, in a mixed population of intergraduig spinose and rela- 

 tively smooth shells, inversion has run riot in both degrees and numbers, and is itself "crowded 

 back" in development from the nearly adult or adult stage to the class 2 or 3 stage. Thus, 

 shells which were spinose at the class 1 and 2 stage become at class 2 and 3 less spinose or per- 

 fectly smooth. The degree of inversion here reaches the most perfect development found in 

 any locality. Some of these changes are shown on plate 3, figures 51, 60-67, and on plate 43, 

 figure 31. Carination is often AveU defined in these shells of inverted development. 



In group 16, from the vicinity of RogersvUle, much the same conditions are found as in the 

 preceding group, but the relative number of the smoother shells is much larger. A large number 

 of the relatively smooth shells show traces of nodules or spmes, suggesting that they have been 

 contaminated by association with the spinose kind. Some of the relatively smooth shells began 

 to develop as smooth, then become spinose, and again smooth, as is shown in plate 44, figure 25. 

 This same kind of change in ontogeny was found in the Powell River in group 2, plate 29, figure 

 32. Of the distinctly spinose shells some begin as smooth, plate 44, figures 12 and 14, while 

 others are spinose and remain so, as shown m plate 44, figure 2. Inversion is also shown in figures 

 21, 22, 24-27. 



No case of inversion was observed in group 17, where the adult shells are all spinose. Three 

 shells are smooth or with only incipient nodules in the class 3 stage (lot 90). These would 

 probably have become spinose, as other individuals are smooth on the apical parts at a corre- 

 sponding age. 



Inversion does not normally occur in locahties where all the shells are spinose; nor have I 

 observed it in such a shell as loudonensis (which is smooth in the early stages and later becomes 

 spinose) except in the case of one shell (lot 126). This shell apparently begins smooth, developed 

 nodules irregularly, again becomes smooth for about one-half of a whorl, and then developed a 

 keeled spine and continued as a very spinose shell. This shell is thus somewhat transitional 

 between loudonensis and turrita. Inversion is characteristic of locahties where the shells are 

 mainly smooth or undulate, or, in other words, where the smooth and sculptured shells have a 

 chance of crossing. 



The following table has been prepared to show the development of the sculpture in the 

 recognized forms of the genus. The smooth kinds are placed at the upper part of the table 

 and the spinose forms at the lower part. Two inverted examples are also included. 



An examination of the following table will show that as a rule the smooth shells, as powel- 

 lensis, develop spinosity and allied sculpture only late in life. In passing on to the spinose forms, 

 as spinosa, there is a progressively earlier development of the spines from the class 4 to the class 

 1 stage. The embryonic whorls are probably smooth, or keeled in aU forms. The position 

 of loudonensis and its allied variations is remarkable. Loudonensis has a continuous range 



