Cincinnatian and Lexington Fossils 49 



than a third of the distance from the anterior edge. Together 

 with the primar}' phcations, they form fascicles each consisting of 

 three phcations. The primary pHcations are not as prominent as 

 in typical Plecforfhis fissicosta. 



Plectorthis plicatella, Hall. 



(Plate VI, figs. 5 A,B.) 



The types of Plectorihis plicatella, numbered 695-2, and pre* 

 served in the American Museum of Natural History in New York 

 City, were obtained from the Fairmount at Cincinnati, Ohio. In 

 this species, the young specimens usually have simple primary pli- 

 cations only, and specimens of this character may occasionally attain 

 a width of 20 to 23 mm. Usually, however, large specimens show 

 at least a trace of secondary plications at a distance of about 9 to 

 10 mm. from the beak, although these may be still comparatively 

 obscure 13 mm. from the beak. 



If this interpretation of the species be correct, Plectorthis 

 cquivalvis and Plectorthis Ussicosta differ from Plectorthis plicatella 

 chiefly in the distance from the beak at which the secondary 

 plications make their appearance and in the relative prominence of 

 the primary plications compared with the secondary ones. 



The group, Plectorthis plicatella, PL eqviivalvis, PI. Ussicosta, 

 characterizes the typical Fairmount fauna of Ohio, Indiana, and 

 the immediately adjacent part of Kentucky. The southern Fair- 

 mount fauna, characterized by Strophomena maysvillensis and 

 Orthorliynchida linneyi, may be termed the Maury fauna, since it is 

 well represented in the so-called Leipers formation of Maury 

 county, in Tennessee. This southern fauna extends northward 

 across Cumberland river, and central Kentucky, toward the northern, 

 part of the latter state, and northeastward into the Alleghanies. 



Plectorthis plicatella-trentonensis, from the Trenton of Minne- 

 sota {Minnesota Paleontology, vol. Ill, plate 33, fig. 5 to 7), and 

 Wisconsin, is more constant in the absence of secondary plications, 

 even in specimens attaining a length of 14 mm. It appears to be the 

 stock from which the Fairmount species of the Cincinnatian areas 

 were derived. 



Plectorthis dichotoma, Hall. 



The figures accompanying the original description suggest that 

 the type of Plectorthis dichotoma was characterized by about 20 



