62 ^4ug. F. Focrstc 



tions are mentioned as occurring on the flanks, these must have 

 been there in order to match those described in case of the sinus. 

 Ten to twelve pHcations occur on each side of the fold. The an- 

 terior face of the shell, along the line of junction of the fold and 

 sinus, is nearly flat or moderaLcly convex. Compared with Platy- 

 sfrophia crassa. the shell is larger, more gibbous, has a more pro- 

 found and longer sinus, a longer hinge-line, and more numerous 

 plications. 



Among the series of specimens preserved in the James collec- 

 tion in the Walker Museum of Chicago University as types of 

 Plafystrophia cypha, and numbered 2326, only one (plate iv, figs. 

 10 A,B, and plate v, fig. 11) is prolonged conspicuously along the 

 hinge-line ; it possessed twelve plications on each side of the fold. 

 The spine-like projection on one side of the shell extends to about 

 three-quarters of an inch from the beak. The spine-like prolonga- 

 tion at the opposite end of the hinge-line has been broken off. The 

 other specimens can not be regarded as types since in these the 

 prolongation of the shell along the hinge-line does not exceed an 

 eighth of an inch and the number of plications on each side of the 

 fold is 7 or 8 instead of 10 to 12. Although all of these specimens 

 can not be regarded as the original types, they evidently all belong 

 to the same species, if my interpretation of the limits of this species 

 is correct. 



The normal form of the species, to one extreme of which the 

 name Plafystrophia cvpha has been applied, is characterized by the 

 deep sinus and prominent fold, with one prominent plication in 

 the sinus and two on the crest of the fold. There is a strong ten- 

 dency toward the disappearance of the lateral plications of the sinus 

 and fold, which is shown by their comparatively inconspicuous size 

 and their situation half way down the flanks of the fold and the 

 sides of the sinus. Occasionally, one of these lateral plications is 

 entirely obsolete both in case of the sinus and the fold. Less fre- 

 quently, both lateral plications are absent on the fold and also on 

 the sides of the sinus. The number of lateral plications on each 

 side of the shell usually varies from 6 to 8, and increases to 10 and 

 12 as a rule only in case of shells which are conspicuously pro- 

 longed along the hinge-line. 



Shells conspicuously prolonged along the hinge-line and with 

 10 to 12 plications on each side of the fold were selected by James 

 as the type of his species, PlatysfropJiia cypha. 



