396 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.56. 



thickness is accompanied by weakening or loss of the lateral plications 

 of the fold and sinus and those of the lateral slopes next to the cardinal 

 angles. 



The globose variety, Platystrophia unicostata crassiformis represents 

 a phylogerontic development from Platystrpohia laticosta through 

 Platystrophia unicostata, and the many narrow paniciplicate variants 

 of Platystrophia cypTia represent alilce development from that species. 



In Platystrophia acutilirata the shell is so thickened centrally and 

 anteriorly that the space occupied by the soft parts is exceedingly 

 small when compared to the size of the shell. This indicates that 

 this species is the most gerontic of the genus. Both valves are 

 marked by strong growth varices, which in the less specialized 

 forms turn out toward the cardinal extremities. 



PATHOLOGIC INDIVIDUALS. 



In the Arnheim beds of the Ohio Valley pathologic individuals 

 belonging to the Poiiderosa subgroup are quite common. They are 

 large, globose, asymmetrical forms frequently with distorted beaks. 

 There is a tendency toward the loss of the fold on one side in some 

 individuals. In others there is a loss of lateral plications of the fold 

 and sinus, and again the plications of the lateral slopes encroach 

 on the fold and sinus in late neanic or ephebic development. This 

 may be accompanied by loss of plications on the lateral slopes next 

 to the cardinal angles, or this development may take place inde- 

 pendently. 



RECURRENT MUTATIONS. 



Typical specimens of the triplicate group have three plications in 

 the sinus and four on the fold. In nearly every species and variety 

 of the triplicate group there are individuals in which there are a 

 greater number. The plications are added in various positions, but, 

 with two apparent exceptions, they originate by implantation in the 

 sinus and bifurcation on the fold. The addition of plications on the 

 fold follows the same order of appearance as occurs in the sinus. 



Hugo de Vries has shown that some flowers regularly produce 

 certain mutations with characteristics which do not become fixed. 

 He says: 



Such deviations are usually called sports. But they occur yearly and regularly 

 and may be observed when the cultures are large enough. Such a variety I shall 

 call ' ' ever-sporting. ' ' 



The striped larkspur is one of the oldest garden varieties. It has kept its capacity 

 of sporting tlu-ough centuries, and therefore may in some sense be said to be quite 

 stable. Its changes are limited to a rather narrow circle and this circle is as constant 

 as the peculiarities of any other constant species or variety. But witliin this circle 

 it is always changing from small stripes to broad streaks and from them to pure 

 colors. * * * Combining the stability and the qualities of sports in one word, 

 we may evidently best express it by the new term of ever-sporting variety.' 



I Species and Varieties. Their Origin by Mutation, p. 311. 



