Cu m in gs — JSIorph ogen es is of Pla tystroph ia . 3 7 



acutUirata is a much smaller shell than P. lynx of the Lor- 

 raine, yet the thickening of the shell in gerontic stages is 

 greater both relatively and absolutely. The acuminate cardi- 

 nal extremities are so thickened that this region of the shell 

 becomes practically filled up with shelly deposit. The thick- 

 ening of the central and anterior region of the shell is very 

 great (fig. 19, A, 1 and 2). so that the actual room left for the 

 lodgment of the soft organs of the animal is less than in 

 unthickened shells of a much lower index, and less both rela- 

 tively and absolutely in gerontic stages than in ephebic stages 

 of the same individual. Fig. 19, C, of a vertical section from 

 cardinal angle to cardinal angle of a markedly senile individual, 

 will make this plain. The convexity (vertical diameter) of the 

 shell is also considerably greater than in any other type of 

 Platystrophia* the height being in extreme cases 1*5 greater 

 than the length, while in P. lynx the extreme is l'Ol or 

 height and length nearly equal, and in laticosta 1'14. The 

 changes in contour due to senescence are profound, as has 

 already been pointed out. Fig. 19, A, represents a shell in 

 which normal growth was attained at the varix numbered II 

 The cardinal angle at this stage is 5S°. In the latest stage it is 

 79°, and in the early stage represented at 7, it is 76°. Here is 

 -a total change of 39°. 



The retrogressive series mentioned above is produced by the 

 acceleration of gerontic stages, till in such individuals as fig. 11, 

 (?, from the extreme upper Richmond beds at Weisburg, Indi- 

 ana, the acuminate or normal a cut Hi rat a stages come on near 

 the beaks and the adult has the outlines of a normal lynx. In 

 fact, the resemblance between this shell and the true lynx of 

 the same horizon is so striking, that only by a study of the 

 stages indicated by the growth varices can they be distinguished. 



Whether any Silurian Orthid was derived from Platystro- 

 phia acutilirata is impossible at present to determine. Cer- 

 tainly none of the Silurian forms of PI at y atrophia bear any 

 close relation to this type, since they are as persistently biplicate 

 as the latter is persistently triplicate. To be sure, many indi- 

 viduals from the upper Richmond beds have one or other of 

 the secondary plications of the fold and sinus originating very 

 near the apex, as we should expect from the marked accelera- 

 tion of these shells in nearly every particular ; but I have never 

 seen a specimen in which the lateral plications manifested any 

 tendency to disappear, nor in which there is even a suggestion 

 of a biplicate type. The form seems to have perished very 

 soon after its assumption of retrogressive characters. Indeed, 

 the coming in of a strongly molluscan fauna in these late 

 Ordovician beds seems to indicate some radical change in con- 



