28 Cumin gs — Morphogenesis of Platystrophia. 



ons. Another feature indicative of extreme senescence is the 

 thickening of the shell. This has advanced to such an extent 

 in many specimens from the "lynx beds" that the space avail- 

 able for the lodgment of the internal organs of the animal is 

 less than in earlier growth stages. In one specimen, fig. 14, C 9 

 there is a very exceptional thickening of the ventral valve over 

 the anterior margin of the ovarian region. In all cases of great 

 thickening of the shell the muscular pit of the ventral valve is 

 profound and the shell substance at its bottom is often so thin 

 as to be translucent. Fig. 14, Z?, shows one of the most mas- 

 sive shells seen. This shell is 12 mm thick at the anterior rim of 

 the muscular pit. Six laminae parallel to the outer surface of 

 the shell (corresponding to as many strong growth varices) 

 indicate the successive additions to the shell substance. After 

 the first (lowest) of these no forward growth took place, and 

 there was a progressive reduction in the amount of room inside 

 the shell. 



14 



Fig. 14. Platystrophia lynx. A, from Vevay, Indiana ; B and C, from 

 Cincinnati, Ohio, all slightly reduced. A, normal adult individual cut antero- 

 posteriorly in the median plane to show normal thickness of valves ; m, 

 muscular platform of ventral valve. B, extremely senile and greatly thick- 

 ened ventral valve, showing lamination of shell substance and six successive 

 additions on its inner surface after normal adult growth had been attained ; 

 t, tooth ; m, muscular pit ; C, greatly thickened ventral valve, interior view 

 showing profound muscular pit, m ; vascular impressions, s ; and abnormal 

 thickening in the ovarian region, th. A, author's collection ; B, collection of 

 Yale Museum ; C, Dyer Collection, Museum of Comparative Zoology, Har- 

 vard University. 



There is no evidence that this gerontic type of lynx extended 

 its range beyond the "lynx beds," or that it produced any radi- 

 cal from which a new orthid stock sprang. It seems to have 

 become extinct. The succession, so far as concerns the remain- 



Fig. 15. a-e, Platystrophia lynx var. laticosta from the Lorraine, Vevay, 

 Indiana, a-a'", anterior, ventral, posterior, and profile views of the form 

 unicostata, in which the lateral plications of the fold and sinus are com- 

 pletely lost ; b-b'" ', same of an individual which has one lateral plication feebly 

 developed ; c-c" , same of an individual in which the lateral plications of fold 

 and sinus are present only in the umbonal region ; d-d'", individual with one 

 complete lateral plication ; e-e'", normal laticosta type. /-/', tracing from 

 Pander's figure of P. costata ; cj -g' ', tracing from Meek's figure of P. dentata ; 

 h-h' , tracing from deVerneuil's figure of P. ehama. a-e, author's collection. 



