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bulletin: museum of comparative zoology. 



Parastrophia, which, differing in many respects from the original 

 specimens, may perhaps, considering the variabiHty of the members 

 of this genus, be included in the Minnesotan group, as a species rather 

 than a variety. 



Winchell and Schuchert state that their variety is "distinguished 

 in having the length and width nearly equal, the valves more convex, 

 and the plications somewhat more pronounced in the fold and sinus 

 and less numerous in the lateral portions of the shell." No further 

 description is given, but the single specimen figured is large (15 mm. 

 long), has three broad plications on the fold, two in the sinus, and a 

 pair on either side of the fold and sinus. 



The three specimens found at Martinsburg are fully as convex as 

 the Minnesotan specimen, but the largest is only 10 mm. long, and 

 all have more and narrower plications in the fold and sinus. Two of 

 them have four plications on the fold and three in the sinus, and the 

 third, the largest, has four in the sinus. Two of them have two and 

 one of them three plications on each side of the fold and sinus. 



This rather detailed description is given because of the increasing 

 necessity of more definite knowledge of the limits of variation, and 

 the possibility of distinguishing species of Parastrophia. 



Parastrophia hemiplicata Hall. 



Airypa hemiplicata Hall, Pal. N. Y., 1847, 1, p. 144, pi. 33, fig. 10. 



To assist in a study of Parastrophia hemiplicata which must ulti- 

 mately be made, it is worth while to record the characteristics of any 

 specimens whose horizon is definitely known. Fourteen specimens 

 were found in the lower thirty feet of the Trenton, but a number of 

 these were too poorly preserved to yield any satisfactory information. 

 The following table shows the principal characteristics: 



