NO. 2.] SNAILS OF THE GENUS 10— ADAMS. 61 



shell as an example of this. In group 2, in a community of smooth and spinose shells, many 

 primarily smooth individuals, as shown in plate 29, figures 22, 23, 24, 25, 32, 37, and 39, developed 

 spines before maturity, and later became less spinose or entirely smooth and thus show inver- 

 sion. The expression of inversion is thus conditioned in part by the early acquirement of 

 spinosity, or we may say spinosity has been crowded back in ontogeny, and permitted time 

 enough to again suppress it. In group 3 the erosion of the shells is so great that it is more dif- 

 ficult to draw safe conclusions on this point. This group, as a whole, is more spinose. Those 

 that most nearly approach the smooth shells, as on plate 30 (mainly on the lower half of the 

 plate) do not show so clearly that they may have been spinose, and later on lost some of this 

 character. One individual in the group, however, clearly showed inversion. In group 4, the 

 approximately smooth shells are more in evidence, plate 31, and most of them have nodular 

 spines. Clearly, then, the relatively smooth shells are becoming spinose at about the class 3 

 stage, and therefore before maturity. A few shells in this group are entirely smooth, as in 

 figure 39, and one became spinose very late in life, figure 30, and others appear to have been 

 spinose onward from the class 2 stage. One individual in lot 180 (No. 52) shows inversion. 



In group 5, from the lower Powell, plates 32 and 33, the presence of smooth shells is still 

 evident. A perceptible degree of loss of spinosity is indicated on plate 32, figures 36 and 39, 

 and in several specimens in lots 28 (Nos. 2, 5) and lot 29, which are not figured. The well- 

 preserved condition of the apices shows that spinosity is developed very early in some shells, 

 as on plate 33, figures 2, 5, 13, 21, and 24, and much later in others, figures 3, 11, and 35. Inver- 

 sion is clearly shown in a few individuals in lots 37 (No. 33) and 38 (Nos. 2, 6). 



In the Clinch, in group 7, from St. Paul, Va., although the shells are both smooth and 

 spinose, spinosity is mainly developed only on the last or adult whorl. There is a suggestion 

 that occasionally incipient nodules and spines are present on the whorl preceding the last, as 

 in plate 35, figures 4, 26, and 29. In several smooth shells, after they had developed nodules, 

 they ceased to do so (in lots 11 and 14), as is shown in plate 3, figure 19. Spinosity has been 

 "crowded back" in these shells. 



In group 8 the shells are mainly spinose, ■ and frequently develop spines before maturity, 

 plate 36. The smoother shells seldom change from greater sculpturing to less, but two shells, 

 plate 36, figures 52 and 55, were undulate or smooth, then became distinctly undulate, and later 

 practically smooth. In lot 164 (No. 5) a very spiny shell changed from spines to a long, strong 

 keel which culminated in a spine, and after a varix had been formed it was continued as an 

 undulating keel. In lot 168 (No. 8) a shell was smooth, then became undulate and nodulate, 

 and finally nearly smooth; in lot 165 a corrugated and nodulate shell developed an undulating 

 keel. 



In group 9, from the vicinity of Clinchport, lot 55, a few individuals show some degree of 

 inversion. The spinose condition changes to a very strong keel and traces of nodules on the 

 last half of the body whorl (Nos. 6, 106); and in others (Nos. 52, 83, 161) inversion is shown 

 in different degrees. One example of this is figured on plate 37, figure 33. 



In group 10, from Kyle Ford, a few shells show some reduction in the spines on the body 

 whorl, and a tendency to form keeled spines, or nodules. 



In general, then, the changes in the Clinch are similar to those in the Powell, the down- 

 stream shells show an earlier development of spines than in the headwaters and they also show 

 some inverted development, but not to such a marked degree. 



In the Holston, in group 12, from Saltville, Va., young shells are few in number, and on 

 these as also upon the apices of the older shells nodules . may develop very early, at the class 

 1 or 2 stage, plate 40, figure 18. Well-defined nodules are present on some individuals at the 

 class 3 stage, as, for example, in figures 11, 16, 17, and 18, and this nodulation is less developed 

 on the last whorl in many cases, as in figures 13, 17, 18, 23, 26, 29, and 33, and in several clearly 

 defined examples which are not figured. Nodulation is sometimes irregular, and not definitely 

 limited to the last whorl, as in the PoweU, and in this respect they show affinity to the shells 

 of the Upper Clinch. In lot 111, from Holston Bridge, inversion was observed in one shell. 



