50 Ajig- P- Focvstc 



primary plications aliernating witli an equal number of secondary 

 plications, except possibly at the postero-lateral angles of the shell. 

 The secondary plications are distinctly less conspicuous and are 

 added about half way from the beak of the anterior margin. The 

 grooves between the primary and secondary plications are relatively 

 wide, and are distinctly marked by concentric striations. 



Specimens possessing these features may be selected from 

 among the numerous individuals belonging to the broad form of 

 PIccforthis cquk'ak'is found in the upper Fairmount, west of Dills- 

 boro station, in Indiana (plate V, fig. i6 A, B, C, D). In these 

 specimens both the brachial and the pedicel valve appear less convex 

 than in the broad form of PlatystropJiia cquk'alz'is which abounds 

 at Cincinnati, but it must be remembered that Hall compared his 

 Plccforf/iis dichotoina not with this broad form but with the narrow 

 individual used by him as a type. Compared with the type of 

 Plcctorthis fissicosta, the plications appear more numerous, and the 

 entire shell less robust. Similar specimens occur at Cincinnati. 



In the original description of PIccforthis dichotoina, however, 

 the number of primary plications is given as 26 and these are 

 stated to bifurcate uniformly half way from the beak to the an- 

 terior margin. The accompanying illustrations suggest that the 

 number 26 may have been a misprint for 20, but as a matter of 

 fact the number of primary plications frequently reaches 24 to 26 

 when there are no intercalated secondary plications near the pos- 

 tero-lateral angles. The emphasis on the regular dichotomous 

 branching of the plications does not assist much in view of the fact 

 that the increase in the number of plications is always by inter- 

 calation and not bv dichotomous branching, and this intercalation 

 is distinctly illustrated by the accompanying figures. The type of 

 Plcctorthis dichotoina has been lost. 



Plectorthis jamesi, Hall. 



(Plate II. tins. A.B.) 



The description of Plcctorthis /(////'W;' appears to have been 

 based upon the series of specimens, numbered 4489, preserved in 

 the American Museum of Natural History in New York City, and 

 there labelled as the types. Among these, the largest specimen was 

 figured by Hall and Whitfield, in the Ohio Paleontology, vol. 2, 

 plate I, figs. 21, 22, as the largest one of the type specimens. It 

 has about 2t, ]Jrimar^• plications, increasing to about 50 at the 



