30 



Cumings — Morphogenesis of Platystrophi 



hi. 



der of the Ordovician of this country was, as we shall see, taken 

 up and passed on by the laticosta and costata types. 



The fact that a more nearly normal form of lynx survived 

 to the close of the Ordovician (Madison beds) has already been 

 pointed out. This form seems to represent the final expression 

 of the type — a shell, primitive in many respects, though of 

 large size ; and undergoing a last attempt to accommodate itself 

 to changing conditions. 



Platystrophia laticosta* — This is one of the most interesting 

 and one of the least understood types of Platystrophia. It 

 seems to be confined to American faunas, and occurs here only 

 in the Cincinnati group of the Ohio valley, where it ranges 

 through the Lorraine and reappears at the base of the Rich- 

 mond formation. 



Meekf has admirably described this variety and little can be 



Fig. 16. A and C, Platystrophia laticosta from Vevay, Indiana ; x %. B, 

 P. costata from Cincinnati, Ohio ; x %. A, shows growth stages I-V separ- 

 ately drawn (6-/), and anterior (g) and posterior (h) views: note the disap- 

 pearance of the right hand jnication of the sinus after stage V. B, shows the 

 initiation of a second and third plication in the sinns at stage II (a,, b) and 

 their disappearance at stage III (a, c). C, is a gerontic individual of P. lati- 

 costa ; note the increase of the cardinal angle after stage I, and the reduction 

 in the height of the fold, and strong growth varices (c, d). A and C, author's 

 collection ; B, Yale collection. 



added in regard to its adult characters. Its minute study, based 

 upon hundreds of individuals, has, however, developed unsus- 

 pected relationships, which I shall now point out. Fig. 15 gives 



* James, Cat. L. Sil. Foss. Cincinnati Group, 1871, p. 10. 

 fPal. Ohio, i, 1873, p. 116, pi. 10, fig. 4. P. chama deVerneuil (not P. 

 costata Pander) may be related to P. laticosta. 



