64 MEMOIKS NATIONAL ACADEMY OF SCIENCES. cvoL.xn. 



even at the class 2 stage, and some degree of inversion may be shown. The relatively smooth 

 shells are evidently contaminated by the spiaose ones. Relatively smooth mature shells are 

 almost absent from the lower Clinch, and occasionally inversion is found. 



Group 2 in the Powell may be compared with group 8 in the Clinch, and they show similari- 

 ties in their transitional stages from the smooth to the spinose shells. Relatively smooth shells 

 reach to the mouth of the Powell, but in the Clinch the corresponding poiat is at about Cliach- 

 port. 



In the Holston the headwater sheUs, group 12, are smooth or undulate when mature, and 

 they show the undulate tendency more than the corresponding sheUs in the Powell and Clinch. 

 Furthermore, undulations become weU pronounced before maturity, at the class 3 stage, in 

 many individuals, while others remain smooth. A few show inversion. Downstream, group 

 13, the undulations become more pronounced, and at the class 1 stage are smooth or corrugated, 

 and a few remain almost smooth, but as a rule they progressively develop the corrugations. 

 These corrugations have been crowded back so that they are well defined at the class 2 stage, 

 and a few individuals show inversion. This is a transitional group between the smooth and 

 spinose shells, and yet the transition is very different from that in the Powell or Clinch. 



In group 14 the shells are distinctly spinose, rather than undulate. At the class 1 stage 

 the apices show that the shells were undulate or spinose, or smooth. A shell may even reach 

 the class 3 stage smooth, and then develop the normal degree of spinosity for the group. These 

 shells also show inversion. Groups 15 and 16 form quite a unique series of shells. They are a 

 mixed series with almost every possible intergradation between smooth and very spiny shells, 

 and even mixtures of smooth and spuiose whorls on the same shell. The young are corre- 

 spondingly variable. Inversion has h^re reached its extreme development. In the Holston 

 there are thus two transitional series of shells between the smooth and spinose, and the transi- 

 tion has been made in a different way in each case. Downstream from this point the character 

 of the young shells and their development is greatly changed. In group 17, from Cobb Ford, 

 at the class 2 stage, the young are smooth or spinose, and these at the class 3 stage are all 

 spinose. These two types of young extend down the Holston to its mouth, in the Lower French 

 Broad, and in all of the Tennessee proper. From these two kinds of young, spinose adults 

 develop, the one with the smooth shell become loudonensis and the other spinosa and its allies. 

 Inversion has not been recognized in these shells. 



The shells of the Nolichucky are spinose throughout their postembryonic development, 

 and do not show inversion. Inversion therefore appears to be confined to those localities in 

 which relatively smooth shells are present with the spinose. A common form of development 

 among the transitional shells is for a young smooth shell to develop spines at the class 2 or 3 

 stage, and to continue spines to maturity. Similar shells are found in abundance in group 2 

 in the Powell and group 7 in the Clinch. In the Lower Holston and French Broad, and in the 

 Tennessee, this kind of development is found in the form loudonensis, which is smooth up to 

 the class 3 stage, and then develops the longest spines in the genus. 



The development of the shells in different streams and locaUties is thus seen to differ as 

 much as the adult shells do, and further the transitional stages of intergradation, instead of 

 being uniform, show a similar amount of individuahty. 



A tabulation of the different forms of development of sculpture shows that in advancing 

 from the smooth to the spinose shells there is a progressively earher development of the sculp- 

 ture from approaching maturity backward to an earlier and earlier stage so that in the extreme 

 spinose shells the entire post-embryonic stages are spinose. The spinose shells thus show an 

 abbreviated development. 



Judging from ontogeny, particularly the smooth character of the embryonic whorls, the 

 normal development of the spines and other sculptural features, and the dominance of the 

 relatively smooth shells over the undulate and spinose in inverse development, the smooth sheUs 

 are the nearest living approximation to the ancestral form from which all the forms have proba- 

 bly been derived. 



