Cuming s — Morphogenesis of Platystrophia. 33 



P. lynx. In fact the gerontic laticosta, in almost every feature 

 in which it departs from the normal adult type of the variety, 

 approaches P. lynx. 



In another place* I have pointed out the intimate con- 

 nection between Platystrophia laticosta and P. lynx. The 

 examination of many thousands of- specimens of these forms 

 has failed to bring to light any character which does not show 

 transitional stages from one to the other. The relatively 

 greater strength and smaller number of the plications of lati- 

 costa is its most constant character, and with this is usually 

 combined auriculation and prolongation of the cardinal extrem- 

 ities : the latter character is, however, by no means uncommon 

 in P. lynx. It seems best therefore, to the writer, to consider 

 laticosta as a variety of lynx, transitional between it and the 

 species acutilirata. 



For comparison with the uniplicate laticosta I have inserted 

 (fig. 15, h) figures of P. chania traced from deVerneuil's 

 figures. f That author says (op. cit.) he succeeded in obtaining a 

 complete series of transitional forms between the small narrow 

 type of chama (= costata Pander) and the acuminate type 

 figured. As I have not seen any specimens of P. chama, 

 deVerneuil's statement must be taken as indicating the relation 

 of these two forms ; though I strongly suspect that chania 

 may bear no closer relation to costata Pander than does the 

 uniplicate laticosta. The latter may, indeed, be the form to 

 which it more nearly corresponds, 



Platystrophia acutilirata. \ — Another type, found only in 

 American faunas, is Platystrophia acutilirata Conrad, which 

 is confined to the Richmond beds of the Cincinnati group.§ 

 This species presents the most remarkable similarity to a 

 Spirifer of any of the types of Platystrophia. The breadth 

 may become as much as three times the length ; and the car- 

 dinal extremities are frequently as acuminate as those of an 

 average Spirifer ?nucronatus,\\ yet between these extreme forms 



* This Journal, July, 1902, p. 14. 



f Geol. de la Eussie, 1845, pi. v. figs.l&, lc. 



X Conrad, Jour. Acad. Nat. Sci., Philadelphia, viii, 1842, p. 260. 



§ Keyes lists this form from the Hudson shales (Cincinnati) of Louisiana, 

 Missouri. As he gives no figure or description, it is uncertain whether his 

 specimens are really referable to acutilirata or belong to the acuminate type 

 of lynx. See Keyes, Geol. Missouri, vol. v, Paleontology, 1894, p. 66. 



|| Prof. H. S. Williams has called my attention to the fact that Atwater's 

 specimen of S. pennatus came from a locality in Ohio to which examples of 

 Platystrophia acutilirata might have been transported by streams. While 

 I do not believe that Spirifer pennatus is a Platystrophia acutilirata. the 

 possibility of this being the case together with the total inadequacy of At- 

 water's description and figures, would seem to warrant abandoning his name 

 altogether and returning to the well known name of mucronatus for this 

 species of Spirifer. See Atwater, this Journal, ii, 1820, p. 244, pi. I, figs. 

 2,3. 



Am. Jour. Sci.— Fourth Series, Vol. XY, No. 85.— January, 1903. 

 3 



