LOGAX: FREEZE-OUT HILLS OF WTOMIXG. 129 



Ostrea dettAa. n. sp. Plate XXVIII, fig. 7. 



Shell ( left valve > very large, subquadrate, test very thick. The beak 

 is truncated and possesses a large quadrangular area which is not de- 

 pressed. The anterior margin is straight while the dorsal and the 

 ventral borders are convex. The posterior mai^in is unknown. The 

 valve is moderately capacious. The exterior surface is - " sud 



nearly flat. The ventral bDrder has about one-half thr • -~ ot 



the dorsal, which is very thick. 



Dimensions : The full length of the sheil is u: Tne A-idth 



exhibited by the largest fragment is 60 mm. : " -mess of the 



test, 10 mm. 



Geological horizon : This species is found in -- "" '^ : 



it is ass«3ciated with Osfrea sttigileeula and P_ 



LocaKty : The Freeze- out Hills, Wyoming. 



Ee marks : This species is not abundant and no complete speci- 

 mens were found. Severed fragments were observed, however. 

 Ot<trea comoensi:*. n. sp. Plate XXVm. fig. 9. 



Shell (left valve) small, capacious for size of shell convex; test 

 thick; the anterior margin acute; the posterior margin somewhat 

 wavy, irregular ; ventral border concave ; dorsal border convex, wavy. 

 The interior of the shell is smooth. The muscular impression is 

 large and projects above the surface of the shell. The ligmental 

 area is quadrangular. The exterior of the shell is lamellose. 



Dimensions : Length. 40 mm. ; width. 21 mm. ; height. 3 mm. 



Geological horizon: This species also occurs in the ! — ^ ^ :t of 

 stratum No. 14. where it is associated with the above -n?.: - ;ies 



Locality : The Freeze-out Hills. Wyoming. 



Remarks : Associated with the above is the fragment shown in 

 Plate IV. iig. IS. This fragment evidently belongs to a new species. 

 but as the essential parts are wanting its foim and description can- 

 not be given. 



Selemnifes densHS Meek. Plate XXTI. fig. 9. 



Guard medium in size, conical in general outline, although the proxi- 

 mal end may be nearly straight. The distal portion t: -_- 

 a point. In cross-section the guard is from oval to su „ — — 

 guard substance is thick and bears a distinct prismatic structure, which 

 radiates in sharp but divergent rays from a line coincident " 

 center of the phragmacone and subcentrally placed. A cros^ - — 

 of the guard shows fine concentric lines of er»3wth. The phragmacone 

 in varying length expands from below-; :.d is slightly curved. 



The shell substance becomes thinner to~ -:- :_- proximal end. The 

 surface of the guard is sometimes punctate but is usually smooth. 

 The guard is composed of prismatic layers, which ar^ separated in 



