LOGAN: FREEZE OUT HILLS OF WYOMING. 123 



forms seems to consist wholly of a siibcentral carina and almost par- 

 allel, slightly radiating lines of growth. This may be an apparent 

 variation in the young of the above-described species which does not 

 find expression in the adult forms. 



Tiiuci'edia }ua{jna. n. sp. Plate XXX, fig. 1. 



Shell large, subovate in general outline. The beaks are promi- 

 nent, inflated, incurved, and project a short distance beyond the 

 cardinal line. They are placed j)osteriorly with regard to the cardinal 

 margin. The hinge line is straight and has a long extension anterior 

 to the beaks. The dorsal border is slightly convex and rounds into 

 the adjacent borders. The posterior border is more broadly curved 

 than the anterior. The anterior portion of the shell is crossed by a 

 transverse depression, which originates near the beak and gradually 

 exjaands toward the basal border. The dorsal border of this depres- 

 sion forms a rather sharp ridge, which is more pronounced near the 

 beak. The surface of the cast is marked by broad bands and fine 

 concentric lines of growth. 



Dimensions : Length, 45 mm. ; width, 38 mm. ; height, 10 mm. 



Geological horizon : This species is found in stratum No. 13 of 

 the section, where it is associated with the fossils already mentioned 

 as characterizing that horizon. , 



Locality : The Freeze-out Hills. Wyoming. 



Remarks : This shell is so much larger than T. hulhosa Whitf ., and, 

 as there ajjpear to be no intermediate forms, I think it must consti- 

 tute a new species. It is not so abundant as the latter. 



Camptonectes hellisti-iatus Meek. Plate XXX, figs. 7, 8. 



Shell large, circular or orbicular in outline, moderately convex. 

 The hinge line is short and straight. The posterior ear is about one- 

 half the size of the anterior ear. The beak is obtusely angular. The 

 surface of the shell slopes gradually back from the beak and attains 

 its maximum convexity at about one-third the distance from the beak 

 to the posterior margin. The surface of the shell is almost smooth, 

 but possessing scarcely discernible radiating stri^, which are crossed 

 by several moderately strong concentric lines of growth and finer striae. 

 The left valve is rather more convex than the right. The anterior ear 

 of the right valve bears a deep notch, which separates it from the body 

 of the shell. 



Dimensions: Lenth, 50 mm.; width, 40 mm.: height, 8 mm. 



Geological horizon : The greenish-colored sandstone of No. 14 of 

 the section bears numerous representatives of this form. 'It is asso- 

 ciated with Camptonectes extenuatus. 



Remarks : The fauna of the hills has numerous representatives of 

 this species. It is also well represented in many other localities in 

 the interior. 



