124 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



stone of the Colorado formation at Bear River city, Wyoming* (No. 12 

 of Meek's Sulphur creek section), where they are associated in the same 

 layer with Inoceramus labiatuSj Cardium pauper culum, Pugncllus fusi- 

 formis, etc. After comparing them with the types of all the forms that 

 are now regarded as varieties of Corbula subtrigonalis I am unable to 

 find any reason for their separation, excepting that some of them are 

 larger than any other known specimens of C. subtrigonalis, and they 

 come from a much lower horizon. The types all came from strata 

 that have been referred to the Laramie, G. subtrigonalis and C. perun- 

 data from the mouth of Judith river, C. tropidopJiora and Corbiculaf 

 crassatelliformis from the Bitter creek series of southern Wyoming. It 

 should be stated that some examples in the Bear River city collections 

 agree perfectly with the varieties described as C. perundata and C. 

 crassatelliformis. Whiteaves reports the occurrence of the species in 

 the Belly River series, which is supposed to hold the position between 

 the Colorado and Montana formations, and I have found it at Coalville, 

 Utah, in the "third ridge," above the Carleton coal bed, which is 

 referred to the Montana formation. The species therefore seems to 

 range through nearly all of the Upper Cretaceous, from the Colorado 

 to the Laramie inclusive, and from purely marine to brackish water 

 beds. The well-known fact that recent marine representatives of the 

 genus are not essentially different from those inhabiting the almost 

 fresh waters of estuaries and the mouths of rivers makes it less sur- 

 prising that in the course of time a species should pass from one of 

 these kinds of habitat to the other without material change. 



CORBULA NEMATOPHORA Meek. 



PL xxvn, Figs. 3 and 4. 



Corlula nematopJiora Meek, 1873, Ann. Rept. U. S. Geol. Sur. Terr, for 1872, p. 496; 

 White, 1876, U. S. Geog. and Geol. Sur. West 100th Meridian, vol. iv, p. 188, PI. 17, 

 Fig. 7; 1879, Ann. Rept. U. S. Geol. Sur. Terr, for 1877, p. 290, PI. 3, Figs. 4o-d. 



Original description: 



" Shell of about medium size, ovate- subtrigonal, nearly equivalve 

 and moderately convex, with height equaling two-thirds the length; 

 anterior outline rounded; base semiovate; posterior extremity some- 

 what produced and subangular or minutely truncated in outline below; 

 dorsal outline sloping from the beaks, the anterior slope being more 

 abrupt and slightly concave in outline above, and the posterior longer 

 and nearly straight, with a greater obliquity; posterior umbonal slopes 

 more or less angular in each valve from the beak to the posterior basal 

 extremity; beaks rather prominent and placed about one- third the 

 length of the valves from the front. Surface ornamented by small, 

 regular, concentric ridges, or strong lines and furrows, both of which 

 are more distinct on the right valve than on the left, where they are 

 sometimes obsolete. 



