TJTLKS AND AI'.STKACTS OF PAPERS 201 



The results of a study of the hrachiopod genus Plat3'stropliia A\ei'e pie- 

 sented b}^ the author, with lantern slide illustrations. Discussed by 

 Messrs. Ulrich and Schuchert. 



SOME MORPHOLOGICAL VARIATIONS IN PLATYSTROPHIA 

 BY MES. EULA D. MC EWAN^ 



(AJjstract) 



A study of the morphological variations of Platystrophia shows that the 

 Orclovician forms have developed along three lines, two of which the writer 

 calls the Uniplicate and the Bifurate types ; the third has been called the 

 Triplicate type. 



The Uniplicate type has one plication in the sinus and is called P. uniplicata. 

 The Bifurate type has one plication in the sinus in the nepionic stage; later 

 this plication bifurcates. A great many individuals do not go beyond this 

 stage; there are greater numbers in which a plication is intercalated in a 

 median position. This is called P. trentonensis. 



The first member of a third line of development passes through a uniplicate 

 stage. After a short interval of growth a plication is intercalated in the sinus 

 on one side of the median plication. A great many individuals were found 

 which do not go bej^ond this stage in development. Another species adds a 

 second lateral plication in the sinus on the opposite side of the median 

 plication. 



All three types are found in the Trenton ; the triplicate type is characteristic 

 of the Maysville and Richmond. 



The Maysville and Richmond species which were studied fall conveniently 

 into three groups. The Ponderosa Group is characterized by large size ; the 

 Low Fold Group has a long hinge relative to the height and retained the low 

 fold of the nepionic stage throughout its entire life history; the High Fold 

 Group has a long liinge relative to the height and developes at an early stage a 

 high compressed fold on which the plications tend to disappear. 



Seven species are placed in the Ponderosa Group, five of which are new. 

 Three species are placed in the Loiv Fold Oi^oup. P. sudlaticosta n. sp. is 

 shown to be the ancestor of P. clarksvillensis and P. acutilirata, which are the 

 other two members of this group. Four species were placed in the High Fold 

 Gn^oup. Three of the members lose the lateral plications of the fold and sinus. 

 P. unicostata loses them by retardation in development, while P. crassa and 

 P. cypha lose them by obsolescence. This shows that the supposed identity of 

 P. unicostata and P. cypha does not rest on suflacient grounds. Index curves 

 do not support the supposed development of P. cypha from P. unicostata. It 

 has been held that P. acutilirata developed from P. laticosta through P. uni- 

 costata and P. cypha. The stratigraphic position of these forms and the long 

 hinge of P. cypha suggest this. These two species, however, represent the 

 culmination of a development toward the excessive elevation of the fold, 

 accompanied by the loss of the lateral plications of the fold and sinus. It is 

 improbable that forms of this type should give rise to a species which retains 



Introduced by A. W. Grabau. 



