82 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



" I had at first remarked that this species is related to I. involutus of 

 Sowerby, but I was not at that time aware how very closely it is allied 

 to Sowerby's species. After a careful comparison of the additional 

 specimens alluded to, with figures and descriptions of I. involutus, they 

 are found to agree in so many respects that I would not be surprised 

 if a comparison of specimens from these two distant localities should 

 prove these shells to be specifically identical. The only differences 

 that have thus far been discovered between them are the following : In 

 the first place the antero-posterior diameter of I. involutus is less in 

 proportion to the height of its left valve than in our Nebraska shell. 

 Again, the aperture of its left valve is more nearly circular, being 

 slightly higher than* wide, while in the shell under consideration, it is 

 somewhat oval transversely, being slightly wider than high. A more 

 important reason, however, for regarding these shells as probably 

 belonging to different species, is the fact that I. involutus occurs in 

 France and England in the Upper or White Chalk, while our Nebraska 

 shell comes from a formation we have reason to regard as equivalent to 

 the Lower or Gray Chalk. 



" From the same locality and position as those from which the species 

 under consideration was collected, Lieutenant Mnllan's party obtained 

 another somewhat aualogous species, which we have described in the 

 Proceed. Acad. Nat. Sci. Phila. under the name of I. exogyroides. 

 Like I. umbonatus, it has a very gibbous left valve, and probably a 

 nearly flat right valve ; but it will be readily distinguished by the 

 much more depressed and oblique umbo of its left valve. This depres- 

 sion of the umbo gives it a nearly circular instead of a vertically oval 

 outline. 



"Locality and position. — Twenty miles below Fort Benton, on the 

 Upper Missouri ; from the Fort Benton group, or No. 2 of the Creta- 

 ceous series." 



In this country the species has been found only at or near the orig- 

 inal locality and in the Austin limestone of Texas. Dr. C. Schliiter 1 

 regards involute specimens as belonging to I. involutus and retains the 

 name I. umbonatus for those (including I. exogyroides) in which the 

 beaks are not coiled. He states, however, that as all three forms are 

 associated in the same strata both in Europe and America, it will prob- 

 ably be found that the greater or less amount of coiling in the beaks is 

 not an essential specific character. It should be stated that I. involu- 

 tus occurs in Germany, in Schliiter's " Emscher Mergel " and in equiv- 

 alent strata in other parts of Europe, at the base of Senonian, where it 

 is associated with Ammonites texanus and Ammonites tricarinatus. The 

 relative position of these analogous if not identical form, therefore, is 

 not very different on the two continents. 



» Palaeontographica, vol. xxiv, p. 272. 



