INVERTEBRATE PALAEONTOLOGY. 33 



due to the breaking away of thai wing, his specimen being an internal cast. 

 Al least, casts of the interior of the form under consideration, such as that 

 represented by our fig. la of pi. 1G, show even less appearance of a pos- 

 terior winy, and about, as straight (though more oblique) a posterior margin. 

 In that specimen, however, this wing is evidently broken away, as other adult 

 specimens show it to be as long and as angular, and the margin as sinuous 

 under it, as represented in outline by our fig. \b of the same plate. I can- 

 not agree with Dr. Stoliczska in referring such forms to Mcleagrina, whether 

 that group should be regarded as a subgenus under Ptcria (= Avicula), or as 

 a distinct genus. 



Our shell seems also to be related to Aoicula petrosa, Conrad, from the 

 New Jersey Cretaceous; but his type-specimen is not in a condition to afford 

 the means for a satisfactory comparison. 



Locality and position. — Great Bend of the Missouri below Fort Pierre, 

 Sage Creek, Fox Hills, Long Lake, &c. It ranges through the Fort 'Pierre 

 and Fox Hills groups of the Upper Missouri Cretaceous series. 



Ptcria I i n a n i f «» r in i s, var. subgibbosa. 



Plate 26, lag. 12. 



Avicula subgibbosa, M. & H. (1860), Proceed. Acad. Nat. Sci. Philad., 180. 

 Pteria subgibbosa, Meek (1604), Smithsonian Check-list N. Am. Cret. Fossils, 9. 



Although I formerly regarded this as a distinct species from P. lingui- 

 fonnis, later comparisons, with the aid of additional collections, seem to indi- 

 cate that it is most probably only a variety of the same. It differs chiefly in 

 its broad, less oblique form, and more broadly rounded, less produced, pos- 

 terior basal margin. I have not seen specimens of the right valve of this 

 form; but, unless they present some peculiarities, it seems improbable that it 

 is a distinct species. 



Locality and position. — Long Lake ; from the upper part of the Fox 

 Hills group of the Upper Missouri Cretaceous series. 



Ptcria Haydcni, H. & M. 



Plate 1G, figs. 2 a, b. 



Avicula Haydeni, Hall and Meek (1854), Mem. Am. Acad. Arts and Sei., V (u.s.), 382, pi. 1, fig. 5. 

 Pleria Haydcni, Meek (1864), Smithsonian Check-list N. Am. Cret. Foss., 9. 



''Shell small, subrhomboidal, oblique; beak small, pointed, slightly 

 5 n 



