2 ( )u m in <j$ — Morphogenesis of Platystrophia. 



Gerontic* stages: second, a critical discussion of the several 

 adult types variously known as species and varieties of Platy- 

 strophia: and third, a general resume and discussion of the 

 history of the genus and the laws of its evolution. 



The material used in this investigation belongs in part to 

 the author's collection, which contains several thousand speci- 

 mens coming from every division of the Cincinnati Group of 

 the Ohio Valley. In addition the Yale Museum contains, 

 besides many specimens from Cincinnati and vicinity, speci- 

 mens from the Trenton of New York, Kentucky, Tennessee, 

 and Minnesota ; from the Galena of Minnesota ; from the 

 Clinton of New York and Kentucky ; from the Silurian 

 (Niagara) of Anticosti and the Island of Gotland; and from 

 the Ordovician of Wesenberg, Russia. Mr. P. E. Raymond 

 very kindly gave me a series of specimens from the lower 

 Trenton of Crown Point, New York ; and Mr. C. J. Sarle of 

 Rochester, N. Y., loaned me a series from the Clinton of that 

 vicinity. Dr. J. M. Clarke loaned for study specimens from 

 the Ryseclorph conglomerate (basal Trenton) in the vicinity of 

 Albany, N. Y. Mr. Chas. Schuchert of the U. S. National 

 Museum loaned specimens from his private collection and the 

 collection of the museum, representing the following localities 

 and horizons : St. Petersburg, Russia (Ordovician) ; Gotland, 

 Sweden (Silurian) ; Gasport and Lockport, N. Y., Eaton and 

 Dayton, Ohio, and Osgood, Indiana (all Silurian). Prof. R. T. 

 Jackson of Harvard University with great kindness allowed 

 me to study their unrivalled collection of Platystrophia, con- 

 taining many thousand specimens from Cincinnati, Ohio (Ordo- 

 vician). Dr. G. F. Matthew sent me for study his types of 

 Orthis lenticularis from St. John, N. B. To Prof. H. S. 

 Williams of Yale University the writer is indebted for oppor- 

 tunity to examine material from the Ordovician of Arkansas, 

 and for suggestions which his wide acquaintance with kindred 

 subjects renders of especial value. To Prof. C. E. Beecher, 

 who has afforded me every facility and encouragement during 

 the prosecution of this work, the author is under deep and last- 

 ing obligations. 



Development of Platystrophia.\ 

 I. Nepionie stages. — The youngest individuals seen (fig. 1), 

 have a breadth of l mm and a length of 066 mra . They repre- 

 sent an early nepionic stage. They are markedly transverse, 



* For a classification of the stages of growth and decline as applied to the 

 Brachiopoda, the reader is referred to Beecher's paper on the Development of 

 the Brachiopoda, this Journal, vol. xliv, 1892, pp. 150-154, pi. i. The 

 substitution of the forms Neanic, Ephebic and Gerontic f or Nealogic, Ephebolic 

 and Geratologie is in accordance with present usage. See Hyatt, Thylogeny 

 of an Acquired Characteristic, Proc. Am. Phil. Soc, xxxii, pp. 390-397. 

 See also Proc. Bos. Soc. Nat. Hist., vol. xxvi, pp. 93-108. 



f Schlotheim's original description of Terebr'atulites (= Platystrophia) bifo- 

 ratus is so difficult of access that it is here quoted in full : 



