No. 2] SNAILS OF THE GENUS 10— ADAMS. 55 



to wear away the shell, even occasionally to the extent of causing perforation. It is also possible 

 that algae are a factor in the destruction of the apex (cf. Collins: Erythea, vol. 5, p. 95, 1897). 

 The slow rate of growth, with approaching old age, probably accounts for the large number 

 of varices found upon the last whorl. If each of these periods indicates a season's growth, the 

 shells possibly reach the limit of growth in seven or eight years. In some mature shells the 

 spines appear to be worn, and a part of the shell is even polished where the shell has been 

 pulled over the substratinn upon which the animal crawled. 



In these lots, carination and undulations, nodules, or spines are almost, at least generally, 

 confined to the class 4 stage in powellensis, although they are much less frequent upon the 

 preceding whorl of the class 3 stage, particulai4y in shells approaching lyttonensis. An excep- 

 tional feature of this upper portion of the PoweU River is the abundance of the immature shells. 



Passing progressively downstream, the next immature shells were found in lots 37 and 38 

 (group 4), collected between McHenrys Ford and extending downstream to Powell River station. 

 These shells were represented by two specimens, class 2 in size, lot 38. One is of the almost 

 smooth kind, plate 3, figure 10, and the other is distinctly spiriose at an early age, figure 11. 

 There are thus degrees of spinosity in this lot, and these young are entirely different from those 

 found farther upstream in the Powell. These shells have become slightly spinose late in the 

 class 2 stage, as in figure 10, or distinctly spinose early in class 2 stage, as ia figure 1 1 . Two shells 

 in lot 37 show similar differences, except that in one the shell was carinate before the spines 

 developed. Upstream when spines are developed, it is, as a rule, late in the class 3 stage or even 

 late in class 4 stage. Class 3 is, however, abiindantly represented in these lots, and they show 

 practically all transitional degrees of intergradation between the smooth and spinose types 

 of shells; a condition also shown in the adult shells. Both relatively smooth and spinose adults 

 occur in these same lots, plate 31. Some, apparently adults, or nearly so, have remained smooth 

 imtil the last whorl was formed and then became quite spinose, as figures 30 and 40. 



Lots 28, 29, and 30 (group 5), from the lower PoweU, contain only a small mmiber of imma- 

 ture shells, and they closely approximate the two kinds found in. lots 37 and 38. A few shells 

 belong to class 2, but the majority to class 3, plate 3, figures 12-16. No representatives of the 

 absolutely smooth shells are found among them, although figure 15 is relatively so (and com- 

 parable with No. 10). A very few relatively smooth adults were found in group 5, plates 32 

 and 33. 



A shell of the class 3 stage in lot 28 (No. 28) is distinctly undulate at the class 1 stage. 

 These are incipient spines for they are progressively larger in the later whorls. In lot 29, from 

 Greens Ford, two young shells have the purple pigment, which characterized lo lurida Reeve. 

 In some of the young shells there is shown a considerable variation in the degree of overlapping 

 which the last whorl makes upon its predecessor, and this exposes the spines to a variable degree. 

 In lot 28 the spines are well exposed, as is shown on plate 3, figure 13, and a similar exposure 

 of a keeled shell is found in lot 29 (No. 202). 



A series of shells from the lower Powell, in Union County, Term., in the collection of the 

 late Mrs. George Andrews, of Knoxville, contains numerous immature shells. There are spinose 

 young in the class 2 stage. At the class 1 stage some of these shells were apparently smooth, 

 and became undulated and spinose at the class 2 stage. These parts of the shell are frequently 

 eroded and give many shells the appearance of being altogether spinose. A few individuals 

 in this series are smooth at the class 2 stage, and possibly a part of the class 3 stage, before 

 developing the strong spines. 



h. Clinch River. — Immature shells were not found as abundantly in the Upper C^linch River 

 as in the Upper Powell. No young of classes 1 and 2 were found at Cleveland or St. Paul, Va. 

 The young of the Cleveland shells are probably smooth or undulate. Lots 11 and 14, from St. 

 Paul, contained a few individuals belonging to class 3. Some of the large shells have sharp 

 peristomes, indicating that growth was still in progress early in August. The season's growth 

 on some of these thin shells was only a small segment of a whorl. On the apex of some shells, 

 corresponding to class 1 of the young, undiilations are found which are not continued, and the 

 remainder of the shell is smooth. In one case Got 11, No. 93) the sheU was undulate at the 



