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



in size even among these shells. This suggests a breeding season not closely limited in time, or 

 variation in the rate of growth. These shells have seven whorls. The embryonic whorls form 

 the apex of the shell at this stage. 



Class 2. Includes shells that are considered to be the second season's growth. 



Class 3. Includes, usually, shells which appear to closely approach maturity in the smaller 

 rivers. 



Class 4. Includes the largest and oldest individuals which characterize the smaller rivers 

 tributary to the Tennessee. 



Class 5. This class seems necessary for the very large shells from the large rivers, as the 

 Lower French Broad and the Tennessee Rivers. 



It is difficult to group these shells because of the lack of distmctly defined stages. The 

 addition of a third or one-half of a whorl makes quite a difference in the size and appearance 

 of a shell, particularly of the larger specimens. Such a method of growth, by whorls, makes it 

 particularly difficult to sort specimens into classes. In many cases allowance must be made 

 for defective whorls. On the other hand, this form of growth and the preservation of the earlier 

 stages on the older whorls, is a distinct advantage in the study of ontogeny. 



The importance of all records throwmg light upon the ontogeny of these sheUs is so great 

 that all data concerning the occurrence of immature shells will be given in some detail, by streams. 

 The shell will be considered from two starndpoints; first, the normal development of the shell 

 and its sculpture, and second, from the standpoint of its inverse development and abbreviation. 



2. THE NORMAL DEVELOPMENT OF THE SHELL AND ITS SCULPTURE. 



a. Powell River. — The two smallest shells from the headwaters of the Powell, Dryden, Va., 

 lot 41, measure in height, from the apex of the spire to the end of the canal 11 mm. and 12.5 

 mm., respectively; and in width 6.5 mm. and 7.6 mm. They have seven whorls; both show 

 faint carination but are otherwise smooth. (PI. 3, figs. 1 and 2.) These shells were collected 

 September 3, so that there is the possibility of a long period of growth preceding. This size 

 includes shells of class 1 , which are taken to approximate the growth of the first season. Upon 

 older shells this portion may often be recognized by its limiting varix and by a change in the 

 color of the shell, the fresher portion being less stained, eroded, or encrusted. 



After what is considered the first season's growth, comprising class 1, erosion is often so 

 marked and the apex so truncated that measurements of the length of the shell are useless. 

 For this reason the width of the shell or the dimensions of the aperture are better for estimates 

 of age. The estimated second season of growth includes shells of about 11 mm. in diameter, 

 plate 3, figures 3-5. Such shells appear to grow about one complete whorl in a season. These 

 comprise class 2. Very rarely carination may develop at this stage. The third season, or class 

 3, also appears to average about one whorl and gives a dimension of about 15 mm., plate 3, 

 figures 7-9. This size is the most abundant of the younger, or at least the smaller sized shells. 

 The shells from this area Gots 41, 45) which develop carination, undulations, nodulations, or 

 spines do so as a rule, at this stage. It is difficult to make an accurate statement because the 

 smooth shells (powellensis) grade into the spinose shells (Jyttonensis) . The first distinctly defined 

 varix is formed at the close of this period of growth. Carination may or may not precede undu- 

 lations and spines, and may follow nodules or spines. 



The fourth size of shell, class 4, includes those of a diameter of about 17 mm. and over. 

 These include the mass of the adult shell population, and the largest shells. But in all proba- 

 bility, this class includes animals of the most diverse ages, from those which have just reached 

 mature size to very old individuals. As a rule, the rate of growth upon the last whorl appears 

 to be slower than in the previous ones, because from one to four varices, indicating periods of 

 rest or lack of growth, are found upon this whorl. In one case the entire last whorl was the 

 product of uninterrupted growth, but this rate is exceptional. 



Old age is indicated by the thickening of the peristome, the heaviness of the shells, and also 

 generally by the truncation of the apex by erosion and acids. The shells are frequently pitted 

 where the epidermis has been broken and allowed the acids and bombardment of sand particles 



