Cumings — Morphogenesis of Platy atrophia. 9 



The above described mode of origin of the plications of 

 the fold and sinus applies to all American Ordovician forms, 

 with the exception of some from the basal Trenton. It does 

 not apply to Upper Silurian forms nor does it seem to apply to 

 Ordovician forms of the Russian Province. Though I have 

 not been able to obtain specimens of Upper Silurian forms 

 of Platystrophia in the nepionic stage, nevertheless several 

 adult individuals from the Island of Gotland are so perfectly 

 preserved that even the nepionic stages can be studied at the 

 beaks. In the Gotland specimens, precisely as in the Ordo- 

 vician specimens just described, there is at first a median 

 ventral fold and a median dorsal sinus. A little later the 

 ventral fold becomes depressed between the two adjacent pli- 

 cations of the valve, and this median plication of the ventral 

 sinus thus formed, immediately bifurcates, making the two 

 main plications of the sinus of all Upper Silurian forms. On 

 the dorsal valve the two plications bounding the median sinus 

 become elevated, and a third is implanted between them, 

 making the three main plications of the dorsal fold of all 

 Upper Silurian forms. All the Russian Ordovician forms 

 studied also belong to this type, as will be shown in a later 

 paragraph. 



An examination of thousands of specimens of American 

 Ordovician forms, and scores of specimens of Silurian forms 

 reveals no real exception to the above described modes of 

 origin of the plications of the fold and sinus. * Yet some con- 

 fusion seems to exist on this very point. 



In the Geology of Minnesota, vol. iii, p. 456, occurs the 

 following statement in regard to the development of Platy- 

 strophia : " The writers [Winch ell and Schuchert] regret 

 their inability to secure very young specimens of this species 

 for the purpose of determining the ancestors or line of develop- 

 ment. In several immature individuals it has been observed 

 that in the early nealogic [neanic] stage the beaks are strongly 

 elevated, probably erect, and each has a very large open 

 delthyrium, surface smooth at first, but gradually developing 

 eight plications and a mesial sinus in each valve. The sinus 

 in the dorsal valve is bounded by two elevations which become 

 plications, and between them is soon developed a single costa 

 which immediately bifurcates. The four plications increase 

 in strength and become strongly elevated as they proceed to 

 the anterior margin, producing the conspicuous fold of the 

 valve." 



With the first part of this statement my observations are 

 in full accord. In regard to the origin of the plications, 



*Some specimens from the basal Trenton, to be described later, are of the 

 Upper Silurian type. 



