32 CONTRIBIJTIOXS TO CANADIAN' PAL.SOXrOLOGr. 



Pteria (Pseud optera) fibrosa, ileek anl Hayden, Var. 



Plate 4, tig. 1. 



Aricula ? fibrosa, 3[eek and Hayden, ISoG. Proc. Ac. IS'at. Sc. Phil., p. SO. 



Pliolndomija nirrosa, 'M and H. 18.56. lb. 2SC. 



Aricula {Psrudopkru) fibrosa, 3Ieelv. 1S73. Sixth Eep. T.S. Geol. Surv. Terr., 



p. 480. 1S7I). E.ep. L'.S. iTeol. Surv. Terr., vol. 



IX, p. 36, pi. 17, fisrs. 17, a, b, c, d. Whitfield (as 



of 51. and H.). PaLeontolo-y of the Black Hills 



of Dakota, p. 386, pi. 7, fig. 5. 



Shell nearly e.:|uiTalve, strongly compressed at the .side-,, obliquely 

 sub-ovate and about one-third higher than long. Posterior margin 

 sloping downwards and backwards in a broadly-convex, oblique curve 

 from the posterior end of the hinge-line to the narrowly rounded ba-.e : 

 anterior margin retreating obliquely backwards and downwards under 

 the beaks, with a slightly and doubly sinuous outline in some speci- 

 mens and a shallowly sigmoid one in others. Hinge-line short and 

 straight : anterior and posterior wings quite obsolete : beaks small, 

 anterior, terminal, curved inwards and forwards : jjosterior area large, 

 broad, obliquely and sinuously flattened, bounded on e:ich valve by a 

 minute, narrow and moderately prominent plication, which extends 

 from the posterior side of the beaks to the corresponding extremity of 

 the basal margin. 



Surface nearly smooth, but marked with a few, faint and distant, 

 rounded concentric undulations. On the posterior area, too, in addi- 

 tion to the minute radiating fold which bounds it, there are two similar 

 but distant radiating folds, which become obsolete towards the hinge 

 near the outer margin of each valve, and l.ietween the innermost of 

 these and the boundary of the area there is a short and not very deep 

 radiating groove or narrow sinus, which also becomes obsolete towards 

 the hinge line. Character of the interior of the valves unknown. 



Length of the most perfect spe:dm?n, nineteen millimetres; height 

 of the same, thirty-one mm. 



Bow Eiver, below Horse-Shoe Bend, Q. 31. Dawson, 1881 ; two well 

 preservel and nearly perfect casts of the interior of the shell. These 

 specimens differ from the typical form of Pteria {Pseu loptera) fibrosa 

 in the much greater lateral compression of the valves, esp>ecially in 

 the umbonal region, and in their nearly smooth surface. They can 

 scarcely be considered, however, as indicating anj'thing more than a 

 local and rather well-marked variety of that species, of which it has 

 been thought desirable to prepare an original description and a figure. 



