LOGAN : FREEZE-OUT HILLS OF WYOMING. 121 



Variety I, plate XXVIII, figs. 1, 2, 5. 



Shell small, irregular in form, sometimes almost cup shaped. 

 Lower valve deep and attached by its entire surface, which is often 

 nearly circular in shape ; muscular impression large, as is also the lig- 

 mental area. Surface of the valve lamellose or strongly wrinkled. 



Variety II, plate XXVIII, figs. 3, 4. 



Shell small, varying in general shape from oblong to ovate, nearly 

 flat, most capacious near the beak; interior of shell smooth, present- 

 ing a glazed appearance ; ligmental area quadrangular and de^jressed 

 centrally. 



Variety III, plate XXVIII, fig. 6. 



Shell larger than the second variety, ovate in general outline, beak 

 acute, with centrally depressed triangular ligmental area ; muscular 

 impression faint, while in the other varieties it is well marked. 



Dimensions: Length, 30 mm.; width, 20 mm.; height, 5 mm. 



Geological horizon : These forms occur in the lower part of stratum 

 No. 14, and also in a thin stratum of No. 15. 



Locality : The Freeze-out Hills, Wyoming. 



Remarks : Osirea strigilecula has a wide distribution in Jurassic 

 deposits of the interior jjrovince. It has been recognized wherever 

 this formation has been recognized. 



Lhiiff lata '/. n. sp. Plate XXX, figs. 2, 3. 



Shell subtriangular, not large for the genus, capacious, umbonal 

 region gibbous, dorsal portion slightly convex. The beak of the shell 

 is moderately acute, and is turned so as to point in the direction of 

 the anterior border. The hinge line is long, the greater part of its 

 extension being between the beak and the anterior border. The an- 

 terior border, as well as the dorsal, is broadly rounded. The posterior 

 border is nearly straight, passing into the adjacent borders by rather 

 abrupt curves. The test of the .shell is thick. The surface is orna- 

 mented with fine radiating striffi and concentric lines of growth. 



Dimensions : The full length of the shell is unknown, but is i:»roba- 

 bly about 55 mm ; width, 40 mm ; height, 10 mm. 



Geological horizon : This species occurs in the nodules of stratum 

 No. 13. It is not of frequent occurrence, as only a few specimens 

 were obtained. It is associated with Pinna kingi, Pleuromya sub- 

 compressa, Belemnites densus, Pseudomonotis curta, Astarte pack- 

 ardi, Goniomya montanaensis, etc. 



Locality : The Freeze-out Hills, Wyoming. 



Remarks: As no complete specimens of this form were found, its 

 determination is somewhat doubtful. The characters of the beak are 

 somewhat uncertain in the specimen figured. 





