Cumings — Morphogenesis of Platystrophia. 



27 



13 



O c c/ 



Gerontic stages of Platystrophia lynx. 

 — The gerontic stages of this species are 

 alluded to above. In the " lynx beds "* 

 at Cincinnati, Ohio, there is a veritable 

 race of gerontic individuals. Fig. 12 

 represents one of these (from the Dyer 

 collection of Harvard). The hinge-line 

 is relatively very short, the cardinal angle 

 being 125°. The vertical diameter ex- 

 ceeds the longitudinal in the ratio of 102 

 to 100. The shell index is 1-16 ; so that 

 the three dimensions of length, breadth, 

 and height are nearly equal. This 

 extreme gibbosity, combined with so 

 short a hinge-line, produces the peculiar 

 effect seen in fig. 12, a ; that is, though a 

 similar view of P. lynx usually shows all 

 the plications of the slopes, in this case 

 only five out of eight are visible on each 

 slope. Normal adult growth was reached 

 at the varix numbered II After this 

 point the increase was mainly in a direc- 

 tion at right angles to the plane of 

 separation of the valves. This has pro- 

 duced such a degree of incurvature of 

 the beaks that the delthyria are com- 

 pletely concealed, and since there is in 

 this individual no encroachment of the 

 pedicle upon the ventral beak, this organ 



must have been reduced to very small Fm 13 PlatystropMa 

 dimensions or possibly have ceased to i yn x from the upper Lor- 



flinctioii at all. The Changing contour raine, Vevay, Indiana, 



of another individual, in which gerontic Rowing growth varices 



. . . J , ' . ,, & (I-Vl) of a gerontic mdi- 



stages are initiated early in the ontogeny, vidua i an a the successive 

 is shown in fig. 13, of a specimen from stages drawn separately 



Yevay, Indiana, b-g show these succes- ( b ~^- ^ tllOT 's collection, 

 sive stages separately drawn. 



The initiation of gerontic stages early in ontogeny is clearly 

 indicative of the paracme of this type of PlatystropMa. One 

 of the most extreme manifestations of senescence is the 

 obsolescence of the plications ; yet in the present group this 

 begins comparatively early (fig. 12, h I) and in later stages the 

 1st, 2d, 3d and 4th plications from the cardinal angle have 

 completely disappeared and the shell is non-plicate, except at 

 the front margin ; and even here the plications are inconspicu- 

 *See Nickles, Jour. Cin. Soc. Nat. Hist., vol. xx, No. 2, Jan. 1902, pp. 85, 



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