40 Curnings — Morphogenesis of Platystrophia. 



almost to identity in everything except size. 

 Fig. 20, a, is drawn from the ventral beak 

 of an adult Bilobites various from the 

 Lower Helderberg of New York; and fig. 

 20, Z>, is a small individual of P. costata, 

 from Cincinnati, Ohio. An examination 

 of several hundred individuals of B. various 

 has shown that there is invariably present, 

 in well preserved material, a median plica- 

 tion at the apex of the ventral sinus* (fig. 

 J^of^uJZl 20,«). This plication very soon becomes 

 cus, drawn from the obsolete, so that the greater portion of the 

 beak of an adult indi- sinus is without plication of any kind, 

 vidual, x4: 6 small C]early such a ves tigial character is not 



specimen or Platystro- .,i -n rm t m 



phia costata natural without signmcance. lhe median piica- 

 size; c, adult Bilobites tion of the sinus of uniplicate and triplicate 

 various, natural size, types of Platystrophia is a character that 

 is never absent whatever other modifications may affect the 

 shell ; the presence of this persistent character in a derived 

 genus is to be expected, and affords, together with the evidence 

 from development, a virtual demonstration of the derivation of 

 Bilobites from Platystrophia. Moreover, since the costata 

 and unicostata types of the latter genus present the most pro- 

 found modifications of fold and sinus, and since this extreme 

 elevation of the fold must have been related to a marked sepa- 

 ration of the brachia and the latter character have been respon- 

 sible for the production of a bilobate shell, we are justified in 

 considering either costata or unicostata as the ancestor of 

 Bilobites. P. costata is preferred because of its wide distri- 

 bution and greater abundance. It would be interesting to 

 know in what province the early stages in the evolution of 

 Bilobites were passed. No trace of the genus has so far been 

 found in the late Ordovician. It is extremely rare in the late 

 Clinton and Niagara of this country ; but occurs somewhat 

 abundantly in the equivalent formations of the Island of Got- 

 land. Since P. costata is abundant in the European province, 

 we may look there rather than in America for the transitional 

 forms to Bilobites. \ 



The taxonomic value of the term costata need not detain us 

 long. De Verneuil regards P. chama as a variety of P. bifo- 

 rata (?), and in this country the form is also usually placed as a 

 variety. It has been shown that there are in the Cincinnati 

 group abundance of transitional forms between costata and the 



* This characterizes all the other species of Bilobites as well. 



f Mr. C. J. Sarle of Rochester, N. Y., has found a form of Bilobites, similar 

 to B. verneuiliana, in the Clinton of Rochester, N. Y., where, in one layer, 

 it occurs rather abundantly. 



