174 CKETACEOUS PEEIOD. 



and subliemispherical ; a very iudistinctly-defined Tinibonal ridge is to be 

 seen upon some examples, especially near the beak, but in others this 

 feature is Avanting. 



Umbo small, distinctly spiral, making about two volutions, sometimes 

 nearly free, but often very closely curved, giving the posterior side an 

 umbilicated character, sometimes having a very small scar of attachment, but 

 often without such a scar, and always quite symmetrical, or at least not dis- 

 torted, as the beak often is in other species of this genus ; peripheiy of its 

 curve usually extending beyond the hinge-line, but sometimes not. The 

 smaller valve is nearly flat, or slightly and somewhat irregularly concave, 

 suborbicular in outline. Surface of both valves having a smooth aspect, but 

 it is marked with such lines of growth as are common to other genera of 

 shells, and free from the lamination of surface so common in the Ostreidce. 



Diameter of the largest example in the collection, from umbo to basal 

 margin, forty-seven millimeters ; transverse diameter, forty-two millimeters ; 

 depth of the larger valve, twenty-eight millimeters. 



The collections contain numerous examples of this species, the type- 

 specimens of which were obtained by Dr. Eoemer from near San Antonio, 

 Texas. The figure given by Dr. Roemer (loc. cit.) represents the umbo of 

 the larger valve more nearly free than that of any of our examples is ; the 

 umbo in all our examples being closely incurved. In this respect, ours are 

 more nearly like those figured and described by Conrad. 



Position and locality. — Strata of the Cretaceous period ; Linear Plateau, 

 Southeastern Utah. 



Exogyra costata Say, var. fluminis. 

 Plato XVII, iig. 3 a, 6, c, and d. 



Shell rather small, irregularly ovate in outline, somewliat exjianded. 

 The larger valve moderately capacious ; greatest convexity near the middle, 

 from which part the sides slope with less convexity to the basal and lateral 

 margins ; an obtuse, indistinct carina generally appears u,pon the umbo, 

 becomes obsolete as it extends, and finally disappears about the middle of 

 the valve; umlbo small, depressed, distinctly spiral; volutions two or less; 

 a scar of attachment usually obscures the inner volution, but seldom or 



