102 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [buia.106. 



Both Prof. Meek and Dr. White have pointed out the (dose resem- 

 blance of this form to Corbicula ( Veloritina) durkeei, a common species 

 of the Bear Biver formation which underlies the beds containing Cyrena 

 securis. Some individuals of the former have almost exactly the 

 outline of this species, but they are usually larger and thicker shells 

 and the hinge seems to be proportionally stronger and with more diverg- 

 ent teeth. I think, however, that they belong to the same genus. In 

 southern Utah several forms of Cyrena are common near the base of 

 the Colorado formation, where they are associated with Glaucoma coal- 

 villensis, Barbatia micronema, Admetopsis, etc. The specimen de- 

 scribed by Dr. White as Cyrena ( Veloritina) durlceei came from this 

 horizon and might be referred with equal propriety to C. securis. 

 Others associated with them approach the form of Cyrena. infiexa, and 

 still others differ from all those species, though I suspect that they all 

 belong to one variable species. (See PI. XXIII, Figs. 3 and 4.) 



None of the species herein referred to Cyrena are typical examples of 

 that genus. They all seem to belong to Meek's subgenus Veloritina, 

 which he regarded as belonging to Corbicula and which combines some 

 of the features of Corbicula, Velorita, and Cyrena, though, as it seems 

 to me, somewhat more nearly related to Cyrena than to the other two 

 genera. 



Locality and position. — From the coal-bearing sandstones at Bear 

 Biver city and near Hilliard station, Wyoming, probably near the top 

 of the Colorado formation. 



Cyrena ^quilateralis Meek? 

 PI. xxn, Figs. 14 and 15. 



Corbicula equilateral is Meek, 1873, Aim. Rept. U. S. Gool. Sur. Terr, for 1872, p. 495. 



Original description: 



" Shell (as determined from an internal cast) subtrigonal, and nearly 

 or quite equilateral, rather convex ; height about five-sixths the length ; 

 anterior and posterior extremities nearly equally, and rather narrowly 

 rounded; ventral margin forming a nearly semielliptic curve, the most 

 prominent part being at the middle; beaks rather prominent, and very 

 nearly, if not quite, central; umbonal slopes not prominently rounded ; 

 dorsal outline declining subequally from the beaks in front and rear, 

 the posterior slope being convex in outline, and the anterior concave; 

 muscular impressions shallow. (Surface and hinge unknown.) 



" Length, 1.72 inches; height, 1.45 inches; convexity, about 0.92 

 inch." 



This species has never been figured and the type has not been pre- 

 served. Becent collections from the typical locality contain casts that 

 agree well with the description and associated with them, one speci- 

 men retaining the shell, which is believed to belong to the same species. 

 The beak is about halfway between the middle and the anterior end, 



