56 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xu. 



class 3 stage, then became undulate and spinose ; but after the varix was formed, the spines were 

 almost completely dropped and a smooth, thin shell growth was formed, plate 3, figure 19. 

 Undulations and spines develop at the class 3 stage of some individuals, and others remain 

 smooth. The adult spinose shells, as a rule, were smooth at the cUss 2 and much of the class 3 

 stage, so that for some individuals spines indicate approaching or achieved maturity, plate 35, 



From the vicinity of Fort Blackmore, Va., an excellent series of very young shells of class 1 

 was secured Got 51) on August 12, plate 3, figures 24-42. This was the best series of very young 

 found in the Clinch, and was superior to any found in the Powell for the very early stages. As 

 other members of this family are oviparous, it seems probable that this is true for this genus 

 also, but no observations have been made upon this phase of their life history. Like the adults, 

 these young were also found in the rapid water upon shoals. It therefore seems necessary that 

 there be some method of anchoring the eggs, or of depositing them under stones, etc. The 

 smallest individual is 4 mm. in diameter and 5 mm. in height; and the larger mdividuals reach 

 6.5 mm. in diameter and 11 mm. in height. These shells have seven whorls. There can be but 

 little doubt but that these are the young of the season. The conical spire and carination of the 

 smaller shells gives them a certain resemblance to the young of the genus Pleurocera, and this is 

 perhaps of phylogenetic significance. The variation among this class of young is of considerable 

 interest, and shows that caution is quite necessary in any attempt to determine the primitive form 

 within too narrow limits, because there may also have been diversity in the ancestral forms just 

 as there is diversity to-day. The young are horn colored, but one very dark individual, due 

 to the deep purple pigment, was found. This is of interest because this same tendency occurs 

 sporadically in several localities. The carination of the whorls, except the minute apical ones, 

 is a feature which is quite characteristic, yet is variable in distmctness. Perhaps an even more 

 important and variable feature is the tendency to form faint transverse undulations, mainly 

 upon whorls six and seven, but also upon five, A perfectly smooth shell, without kregularities 

 of the surface or undulations, is quite rare. It should be mentioned that these shells come from 

 a region where the adults are primarily spinose, plate 36. 



The nearly adult or adult shells of lot 51 contain many individuals which possess a sharp 

 peristome, thus showing that the growing season contmues at least until about the middle of 

 August in the Upper Clinch. These sharp-edged shells further show that their last period of 

 growth was a short one, involving only a fraction of a whorl, even about a centimeter in linear 

 growth, as in plate 3, figure 19. 



In lot 169, from near Crafts Ferry, there are two shells, one of class 2, plate 3, figure 21, 

 and the second of class 3, figure 22. The class 2 shell is distinctly spinose on the last half of 

 the body whorl, whUe the other has low nodules. In addition, there is a shell smooth at the 

 class 2 stage and spinose at the class 3 stage (No. 5). 



A few young shells were found m group 8 from the vicinity of Fort Blackmore, Va., m lots 165, 

 166, 167, 168, and 170, most of which belong to class 3, and possibly also to class 2 ua a few cases. 

 These shells include not only smooth individuals which were smooth at the class 2 stage, and 

 also those which showed their spinose tendency at the class 2 stage, but on the last whorl (class 3) 

 became spinose. Lot 165 (No. 1) contained a small, smooth shell, class 2, which has faint trans- 

 verse undulations upon the two whorls preceding the last one, plate 3, figure 23. One shell 

 from the lot was irregularly smooth during the class 2 stage, and became feebly undulate and 

 developed two low but distinct spines, and then grew about one half of a whorl with an imperfect 

 keel and low nodules. The possible significance of such shells will be discussed later. The apical 

 whorls of some specimens from Clinchport, Va., were secured from shells which had been pur- 

 posely broken. An examination of these fragments showed that in the revolving growth of a 

 whorl, the upper edge of the aperture varies in its line of contact or fusion with the preceding 

 whorl. In very young shells this line may be just below (away from the apex) the carina and 

 thus leave it exposed; in case of overlapping on the carina there may be the apparent lack of it. 

 In older shells tnis same process may leave undulations and spines either covered or exposed. 

 In case low spines and undulations are hidden, it gives the shell the appearance of having smooth 

 apical whorls. For this reason it is evident that real and apparent smoothness must be borne in 

 mind when referring to the spinosity or smoothness of the apical whorls. It is apparent that 



