Cumings — Morphogenesis of Platystrophia. 121 



Art. X. — The Morphogenesis of Platystrophia. A Study 

 of the Evolution of a Paleozoic Brachiopod / by Edgar 

 Roscoe Cumings. 



[Continued from p. 48.] 



General Observations and Conclusions. 



Platystrophia, as shown by its development and early his- 

 tory is descended from an ancestral Orthis having eight or 

 nine plications on each valve, a ventral fold and dorsal sinns, 

 and erect, high cardinal areas, with large open delthyria in each 

 valve. Such a combination of characters is found in adult forms 

 of the Orthis lenticularis group of the Upper Cambrian. The 

 close resemblance of Orthis lenticularis to the nepionic shell of 

 Platystrophia, together with its high variability and wide dis- 

 tribution, warrant the conclusion that it is the ancestor of the 

 latter genus. 



From this primitive stock was derived in the early Ordovi- 

 cian, a shell with a ventral sinus and- dorsal fold ; and with one 

 plication in the sinus, two on the fold, and four" to six on either 

 side. In, the early Trenton (or possibly before) this latter 

 form underwent a differentiation into two species ; one having 

 fundamentally one plication in the median line of the sinus, 

 representing the continuation of the median ventral fold of the 

 nepionic stage, an cr an additional implanted plication on either 

 side of this primary plication; the other having fundamentally 

 two plications in the sinus arising by the bifurcation of the 

 nepionic ventral fold. These species are P. lynx and P. bifo- 

 rata, respectively. 



From .P. lynx was derived in the Ordovician of America (a), 

 by reduction of the secondary plications of fold and sinus, the 

 variety costala of the lower Lorraine; (b), by elevation of the 

 fold and extension of the cardinal angles, the variety laticosta 

 of the Lorraine ; and {c), by farther extension of the cardinal 

 angles and addition of lateral plications, the species acutilirata 

 of the Richmond ; id), by obsolescence of the secondary plica- 

 tions of fold and sinus in laticosta, was produced the form 

 unicostata of the upper Lorraine. P. lynx attained its maxi- 

 mum development (acme), living in association with the above 

 varieties, in the Lorraine of the Ohio Valley, and in the upper 

 Lorraine passed through a series of gerontic modifications. 

 A nearly normal form of lynx_ survived till the close of the 

 Ordovician, making its appearance in great numbers in the 

 extreme upper part (Madison beds) -of that system. P. acutili- 

 rata passed through a series of retrogressive modifications in 

 the upper Richmond, and became extinct at the close of the 

 Ordovician. P. cosiata, by accentuation of the fold and sinus 

 (bilobation), gave rise to the genus Bilobites, which survived 

 till the early Devonian (Lower Helderberg). 



Am. Jour. Sci. — Fourth Series, Yol. XV, No. 86.— February, 1903. 

 9 



