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



was approached and thus corresponding with those of lot 47. The apical whorls of other shells, 

 plate 50, confirm these differences. Lot 102, from Williams Shoals, contains a few immature 

 shells of class 3, and show both types of young. In lot 103, from Little River Shoals, there are 

 a few small individuals of class 3, the spiny ones predominating. The apical whorls of class 3 

 and older shells clearty show the predominance of the spiny kind, but the number of those with 

 smooth apices is greater than in previous lots from the Tennessee, excepting lot 47. Lot 105, 

 also from Little River Shoals (group 23) is composed mostly of very large and mature or old indi- 

 viduals, whose apices are much eroded, plate 51. The youngest shells of lot 105 are about the 

 size of class 2 of lot 45, from the headwaters of the Powell River. One of these shells is entirely 

 smooth, and two others are carinated and with undulations. Such young are clearly the domi- 

 nant kind; they are loudonensis. Other immature shells (turrita) show indications of spines 

 at the class 1 stage and continued spinose, wliile others pass the class 3 stages as practically 

 smooth shells before developing spines. This group is a mixture of loudonensis and turrita. 



A very large series of shells was secured from Loudon, Tenn. These included lots 126, 

 130, 134, 152, and 189, and contained very few immature shells. In the scale of five sizes the 

 immature shells belong mainly to class 3, Avith a few in class 4. Shells with smooth apical 

 whorls {loudonensis), plate 5, figures 49-53, far outnumber the very few with spiny apices. As 

 a rule, also, the spiny angitremoides shells are of smaller size than the large shells with smooth 

 apices, loudonensis, and belong mainly to class 4. Below Loudon, Tenn., no young five shells of 

 angitremoides were found, although the adults of turrita range far downstream in the Tennessee. 



A few young with smooth apical whorls are in lots 199, 202, and 203, from Chattanooga. 

 Those in lot 199 reached the size of shells in classes 2 and 3 from the headwaters of the Powell 

 before undulations or spines were formed, plate 5, figures 54-55. The apical whorls of lot 202, 

 plate 5, figures 56-58, and lot 203, figure 59, show the same kind of young as lot 199. A similar 

 shell was found at Bridgeport, Ala., lot 186. This was the farthest do^vnstream that I secured 

 living lo. These apically smooth shells are the form loudonensis. No young of those with 

 spinose apices were found below Loudon, Tenn. 



3. ABBREVIATION AND INVERSE DEVELOPMENT OF SPINOSITY. 



In what may be considered the normal development of spinosity or undulations the sculp- 

 ture begins to develop upon a smooth shell and progressively, with age, becomes more spinose 

 or undulate, or spines or undulations may be present throughout the post-embryonic develop- 

 ment. There is still another sequence of development, in which a shell begins normally and 

 after forming spines or undulations, becomes less so, or ceases altogether to form them. This 

 form of development will be called the inverse development of sculpture. 



Previously, brief mention has been made of cases where a shell inversely changed its sculp- 

 ture with age, as in group 16, from the Holston River. These shells were spinose and later became 

 less spinose or smooth. This kind of change may possibly be the result of crossing. It is 

 desirable, therefore, to investigate the occurrence of such shells, among the mixed series, ia 

 order to see how general this tendency is. Of course if the mixing of forms has taken place 

 this would be more likely to occur where both forms are found in the same locality. Relatively 

 isolated examples of one form, occurring in a region where others predominate, would be 

 expected to be favorable to mixing because of the few chances for them to mate with their 

 own kind. 



Do such isolated individuals show in their development variations from those individuals 

 found in homogeneous communities? For example, do smooth shells develop the same in a 

 smooth shell community as they do in a spinose colony ? Answers to such questions should 

 aid in an understanding of the significance of some of the peculiarities of development found ia 

 the mixed communities of shells. 



In the headwaters of the Powell, group 1, in a primarily smooth-shell colony, a few shells 

 develop spines only as they approach maturity or very late in life. In such shells, of course, 

 there is little chance to show inversion, or the cessation of spine formation, and I have not found 

 it in them, unless we consider the formation of sporadic undulations on an otherwise smooth 



