INVERTEBRATE PALAEONTOLOGY. 37 



known in regard to its hinge. A single right valve exposes a little of the 



hinge-margin near the beak; it is narrow, concave, and shows (races of 

 minute transverse and longitudinal stria', somewhat like the ligament-area we 

 see along the hinge-margin of Pteria proper. So far as can be seen, however, 

 it seems to have no hinge-teeth or deep marginal excavation for a partly- 

 internal ligament. 



Of all the Upper Missouri Cretaceous shells yet studied, few, if any, 

 have given origin to so much perplexity in the determination of their affinities 

 as this one. This was due, of course, to a great extent, to the very imperfect, 

 mutilated condition of the specimens, and also, in part, to the peculiar char- 

 acters of the fossil itself. In first describing the species, we referred it, with 

 a mark of doubt, to Avicula, stating that this was done only provisionally, as 

 none but quite- imperfect specimens, giving a very unsatisfactory idea of the 

 entire shell, had been seen. On seeing the description, Dr. Shumard wrote 

 to me that he tiad seen good examples of this species, and that he regarded it 

 as a peculiar oblique form of Pholadomya ; to which we afterward referred it 

 on his authority.* Still later, on examining some additional specimens of the 

 same, as seen flattened in masses of rock, I was led to believe it more prob- 

 ably a peculiar, short, gibbous form of Pinna, particularly in view of the fact 

 that fragments of the shell had previously been observed to show indications 

 of a fibrous or prismatic structure. 



In studying the collections obtained in connection with Dr. Ilaydcn's 

 survey of 1872, I observed two species, which, although differing in form 

 from this, still seemed, on comparison, to present, along with a European 

 Cretaceous species, certain characters in common, warranting their arrange- 

 ment in a single group, very near, if not within, the genus Pteria {Avicula); 

 and for this group I proposed, as has already been stated, the name Pseucl- 

 optera, as a subgenus under Avicula, placing this species in the same. 



Locality and position. — Forks of Cheyenne River, from the upper part 

 of the Fort Pierre group ; and from the Fox Hills group, on North Platte 

 River, near the mouth of Deer Creek. 



* Dr. Shumard may have confounded auother shell with our typo. 



