12 



( 'umings — Morphogenesis of Platystrophia. 



geeted above, it may have been separately derived from the 

 Orthis lenticularis stock. 



III. Epftebic stages. — The ephebic stages begin with the 

 assumption of characters of lowest taxonomic rank (varietal) 

 (see fig 3 : X.1-XVI). During these stages there is no marked 

 change in the number of plications, though these may he 

 added, or in the case of the fold and sinus, subtracted, until the 

 final assumption of gerontic characters. The convexity of 

 the valves increases progressively, and the incurvature # of the 

 beaks becomes more and more pronounced. The pedicle often 



Fig. 5. Very large senile individual of Platystrophia lynx from the upper 

 Lorraine of Vevay, Indiana ; x i^. A, profile ; B and 0, sections in the median 

 plane. Normal adult growth was attained at varix No. I. The successive 

 additions of shell substance are shown in J3, and the probable musculature in 

 C. The pedicle has entirely abandoned its original foramen and encroaches 

 on the umbonal region of the ventral valve ; the apex of the dorsal valve 

 lies in the ventral delthyrium. The shell was partially infilled with very fine 

 silt, probably while still standing in its natural position, with the shell 

 resting on the plane tt'(s). f, pedicle passage ; mm', muscular platforms 

 of dorsal and ventral valves respectively ; a. b, oxidized and unoxidized 

 portions of infiltrated material ; c, cavity lined with small crystals of cal- 

 cite ; C, a., adductor muscle ; d., diductor ; ad., adjusters ; p., pedicle ; c, 

 position of hinge axis. Author's collection. 



encroaches on the apex of the ventral valve, even sometimes 

 in early ephebic stages. All, or any of these changes may 

 affect any one of the forms under which Platystrophia is 

 known. Other modifications affecting the ephebic stages can 

 more conveniently be discussed under the heads of these 

 various forms. 



IV. Gerontic stages. — Old age manifests itself in Platy- 

 strophia by increasing gibbosity, thickening of the shell, 

 pronounced growth varices, and obsolescence of plications, and 

 at times by marked changes in the contour of the shell (shell 

 index). Though senile individuals are met with in all the 

 types of Platystrophia^ lynx and aciotilirata afford the best 

 examples of it, and in many specimens from the upper range 

 of these npecies senescence begins early in the ontogeny. 



