48 Aug. F. Foerste 



Plectorthis fissicosta, Hall. 



(Plate VI, tig. 4.) 

 (Plectorthis tissicosta, type, tliis Bulletin, vol. XIV, pi. IV, fig. 5 A,B.) 



The type of Plectorthis fissicosta, Hall, preserved in the 

 American Museum of Natural History, and numbered 4490, agrees 

 very well with the original description and with the general appear- 

 ance of the accompanying figures. The length of this type speci- 

 men is 16.5 mm., the width is 21 mm., and the thickness probably 

 was about 8.5 mm., but the latter can not be determined accurately 

 since the specimen is crushed. There are 20 primary plications, 

 of which II show the so-called fission of the plications very well, 

 and 2 only poorly. The postero-lateral plications appear narrower 

 and remain simple. The secondary plications are added about 9 

 mm. from the beak. Together with the primary plications they 

 form groups or fascicles, each consisting of three plications among 

 which the primary plications are much more elevated and more 

 conspicuous. In the case of two fascicles, in case of this type, 

 there are 4 plications, instead of 3, present. 



Specimens of this type are rare in the upper part of the Fair- 

 mount bed at Cincinnati. Ohio, and occur as far w'est as Vevay, 

 Indiana. 



Plectorthis triplicatella , Meek, is founded upon a shell pos- 

 sessing the same characteristics, and this name should be dropped 

 for the prior one, Plectorthis fissicosta. Hall. 



Plectorthis fissicosta' is a variety or direct descendant of the 

 normal broad-shelled foniis of Plectorthis equivalvis, of which the 

 narrow specimen used as a type is only an individual variation. 

 Plectorthis fissicosta is characterized by the great prominence of 

 the primary plications, causing them to appear widely separated, 

 especially along the middle parts of the shell. The secondary 

 plications are intercalated at a considerable distance from the beak, 

 thus adding to the appearance of the primary plications being 

 separated by deep grooves. These features are possessed, although 

 in a less accentuated degree, also by the specimen represented by 

 figures 9 f, g, on plate 32, Neiv York Paleontology, vol. i, and 

 there included as one of the forms of Plectorthis plicatello. This 

 specimen, preserved in the American Museum of Natural History 

 and numbered 695-2. is not well represented by the published figure. 

 The secondary plications are not added until within slightlv less 



