112 INVERTEBRATE PALEONTOLOGY. [WHITE. 



CRETACEOUS PERIOD. 

 Mollusca. 



Conchifera. 



Genus OSTREA Linnaeus. 

 Subgenus ALECTEYONIA Fischer. 



Ostrea (Alectryonia) sannionis (sp. nov.). Shell rather small, alate at 

 both sides of the beak, irregularly subquadrate in marginal outline, its 

 longitudinal axis curved, the convexity -of the curve being forward, almost 

 as wide across the alations as at the base, but constricted in the middle ; 

 beaks small, not prominent, directed slightly backward ; lower valve mod- 

 erately convex ; scar of attachment at the beak small or. absent ; ligament- 

 area short, rather broad ; its longitudinal furrow shallow but well defined, 

 transversely striated, and pointing obliquely backward ; posterigr alation 

 narrower than the anterior one, and a little longer than the corresponding 

 alation of the other valve ; muscular scar comparatively large, situated 

 nearly mid-length of the valve and near the posterior margin, curved- 

 spatulate in outline, the broadest end being toward the base of the shell ; 

 upper valve nearly flat, but in other respects corresponding with the lower. 



Surface of both valves marked by the ordinary lines and lamellatioris 

 of growth common to the genus and by numerous creiiulated radiating pli- 

 cations, four or five of which upon each valve reach the base of the shell, 

 giving that margin a coarsely zigzag or toothed condition. The other pli- 

 cations are smaller and die out at the sides of the shell and upon the alations. 



Length from base to beak of a large example thirty-eight millimeters ; 

 breadth near the front the same; across the wings, thirty- three millimeters. 



This is one of the most distinctly defined species of the genus known 

 to me, and numerous examples of it show that it was subject to compara- 

 tively little variation. 



. Position and locality. Near top of Salt Wells Group ; Weber Valley, 

 near Coalville, Utah. 



Ostrea insecura (sp. nov.). Shell rather small, thin, elongate-suboval 

 in outline when adult, suboval or snbcircular when young ; beaks and area 



