16G 



According to Mr. Gabb, " C. truncata is allied to C. Nebrascensis, Owen; 

 but differs in the smaller beaks, proportionally longer form, more 

 oblique truncation posteriori}^ and in being moi-e produced in the 

 anterior basal region." "There is also," he adds, " a difference in the 

 hinge. In C. Nebrascensis the lateral teeth radiate, as it were, from an 

 imaginar}- point, while in the i^re^ent species they are parallel with each 

 Other and witli the uppei- edge of the hinge plate, and their inner ends 

 are bent at a i-ight angle." * Since these remarks were written Mr. 

 Meek has published excellent new desi-riptions and figures of 0. Nebras- 

 censis, and a comparison of these with twenty more or less perfect 

 specimens of 0. truncata from the Vancouver Cretaceous shows that the 

 beaks of C. truncata are fully as large as those of C. Nebrascensis, and 

 that the hinge dentition and outline of the valves are alike in both. 



There are several reasons for supposing that CucuUcea Nebrascensis of 

 Owen, C. Shumardi of Meek & Hayden, and 0. truncata of Gabb, have 

 been separated as distinct species on insufficient grounds, and that all 

 three may be mere varieties of the C. glabra of Parkinson, of which C. 

 fibrosa, Sowerby, is generally admitted to be a synonym. At present it 

 does not seem jiossible to distinguish detached immature valves of C. 

 truncata from those of C. Shumardi or C. fibrosa by any known character, 

 and C. Nebrascensis appears to bear the same relation to C. Shumardi and 

 to C. truncata that C. glabra does to C. fibrosa. In 0. glabra proper, as 

 the name imports, and in C. Nebrascensis, the radiating ribs are absent or 

 nearly so at all stages of growth, but in 0. fibrosa, 0. Nebrascensis 

 and C. truncata they are alwaj's present, though sometimes rather 

 obscure. 



In 1860 Mr. Meek united C. Shumardi to C. fibrosa, but subsequently 

 changed his opinion on this point, on the ground, first, that C. fibrosa " is 

 a Gault and Greensand species," while C. Shumardi " has only been found 

 in beds equivalent to the true chalk ; "y and, secondly, because D'Orbigny 

 states that the right valve of C. fibrosa is the largest of the two, whereas 

 in C. Shumardi the left valve is the largest. But to these arguments it 

 may be replied : (1) that the circumstance that C. fibrosa is known to 

 occur in the Lower Greensand, Gault, and Upper Greensand of Europe 

 is no reason why it should not be found on the horizon of the chalk 

 proper in America; and (2), though D'Orbigny certainly does say that 

 the right valve of C. fibrosa is larger than the left, yet on figure 6 of 



• Paliuontology of California, Vol. I., p. 196. 

 t Report on the Invertebrate Cretaceous and Tertiary Fossils of the Upper Missouri Country, 

 p. 87, 



