THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



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

 of the Evolution of a Paleozoic Brachiopod ; by Edgar 



ROSCOE CUMINGS. 



Few fossils present features of as great interest and import- 

 ance, both to geologists and zoologists, as the Brachiopoda, 

 and among them, few are as familiar as the genus Platystrophia, 

 the " Orthis lynx " of collectors. Wherever Ordovician rocks 

 are exposed the geologist may expect to meet with this ubiqui- 

 tous and characteristic genus. The regularity with which it 

 turns up in collections from new localities is little short of 

 amazing, and perhaps, if familiarity breeds con tempt, accounts 

 for the indefinite and hazy notions current as to its real history 

 and the interrelations of its component elements. 



The writer has made an attempt in a preliminary paper to 

 obtain some measure of the variability of Platystrophia and 

 out of that study has grown a deeper interest in this form and 

 a deeper insight into its history. The present study is based 

 upon the method of investigation of post-embryonic growth 

 stages, which we owe to Alpheus Hyatt and his able exponents 

 of the present day, Beecher, Jackson, Smith and others. I 

 have endeavored to make full use of specimens representing 

 all stages of growth and all the elements of the range and dis- 

 tribution of Platystrophia. The mere study of growth stages 

 has not been deemed adequate unless supplemented by series 

 of adults ; and it is hoped that in this way is escaped the error 

 of depending on one class of evidence. That this method thus 

 supplemented has proved potent in discovering unsuspected 

 relationships among the members of a genus which has 

 attracted widespread attention for nearly a century, will be 

 evident to those who follow the present discussion to its close. 



The order of presentation is, first, the development of Platy- 

 strophia under the heads JVepionic, JVeanic, Ephebic and 



Am. Jotjr. Scl— Fourth Series, Yol. XY, No. 85.— January, 1903. 

 1 



