No. 2.] 



SNAILS OF THE GENUS 10— ADAMS. 33 



Tennessee and French Broad are relatively globular on account of the presence of the shells of 

 loudonensis, while the relatively elongate shells, which abound in the Nolichucky and the 

 Tennessee, are the form turrita and its allies. 



3. VARIATION IN SPINOSITY. 



The variation in this genus, from the smooth shells to the most spiny kinds, is perhaps 

 their most striking feature and one of their most variable characteristics. It should be recalled 

 that nodules and spines, of the character found in this genus, are of rare occurrence in the family 

 Pleuroceridae, but sculptured surfaces, produced by carina, nodules, and low spines are of fre- 

 quent occurrence. The two genera which show the greatest sculptural tendencies are lo and its 

 nearly related genus Angitrema. The smooth shells of lo show httle resemblance to Angitrema, 

 but the spinose shells, particularly the form angitremoides from the Tennessee and the Nohchucky 

 Rivers show much superficial similarity to Angitrema armigera Say. 



The tendency to form spiues is thus seen not to be solely confined to this genus, although 

 it receives its greatest development in it. In order to study the variation in this trait, quanti- 

 tative estimates have been made of it. The last two or three spines formed on the last whorl 

 have been measured. It must be recalled that this whorl may be smooth or show all grada- 

 tions to strong spines. These spines are the latest formed and are thus less Hable to be injured, 

 and are most hkely to give the adult condition. They are also, in general, the largest spines. A 

 serious difficulty, for comparative purposes, is the difference in size of the shells which is due 

 to age; but we have not been able to see how to overcome this difficulty, without at the same 

 time running the risk of an equal or greater error due to the personal equation. At any rate, 

 the curves show the condition of the population at a given place, even if it is occasionally in- 

 fluenced by the inequahty due to age. But in general the definiteness of the results gives con- 

 fidence to their general refiability. No effort has been made to carry the statistical method 

 to its fuU extent. There are several reasons why this has not been done. Only to one famihar 

 with the material can the chances for errors in handling such a collection be reahzed. The 

 spines were in many cases eroded or shghtly injured ; how much, no one can tell. To cast out 

 all such material would soon present serious difficulties of even greater importance than a slight 

 error in spine height, because of the hmited number of specimens that would remaia. Then 

 again, erosion or injury of the spines is not uniformly distributed among the shells. And lastly, 

 the incipient stages of spinosity could not be measured with much accuracy, as, for example, 

 when only one spine was present, or some irregularity of the shell could not be distinguished 

 from a spine, or the spines or nodtdes were too far apart, etc. In locahties where the relatively 

 smooth shells are present many individuals may be spinose and become less so or even smooth 

 on the body whorl. As the spines were measured on this whorl such "inverted" spinose indi- 

 viduals must count as smooth shells. In groups where the smooth and spinose are mixed the 

 smooth shells were counted and placed in the lowest class. I cite such cases to emphasize that 

 caution is necessary in drawing conclusions from such material. 



I have tried to draw conclusions only within relatively safe limits. Undoubtedly the 

 greatest source of error is due to the heterogeneous character of the groups, but as mentioned, 

 I have found no practicable way of avoiding this. 



AU measurements have been made in mm., the tenths having been estimated. These deter- 

 minations have been made by the use of an instrument designed by Dr. C. B. Davenport, which 

 measured the depth of the vaUey between the spines and the distance between the apices of the 

 spines. The spine index is derived by dividing the average height of the spines by the average 

 of the distance between them. 



a. Height of Spine. 



A. POWELL RIVER. 



Group 1. This series of shells were smooth or with low spines, as indicated by the curve, 

 plate 14, No. 1, with its mode at 0.3 mm. 



Group 2 also shows its mode at 0.3 mm., but with a decided skewness toward 1.3 mm. 



