194 CRETACEOUS PERIOD. 



by cons2:)icuoiis lines of growth, wloich are crossed by numerous finer 

 radiating- lines. Besides the latter, there are eight slightly-raised radiating 

 ridges extending from the apex to the border, which leave upon the internal 

 cast faint shallow fmTows, and where the shell is partially exfoliated they show 

 asnarrowlinear grooves in its substance. These gi'ooves or ridges are usually 

 an-anged with considerable regularity as to their relative distances from each 

 other, but sometimes they are less symmetrically an-anged than they are rep- 

 resented in figure 9 a. The margin is usually very slightly emarginate at 

 each of the radiating ridges, and seems to have been elevated a little at the 

 front. 



Antero-posterior diameter, thirty millimeters ; transverse diameter, 

 twenty-five millimeters ; height, about twenty millimeters. 



Position and locality. — Strata of the Cretaceous period ; Gallinas Creek, 



New Mexico. 



Anisomyon centrale Meek. 



Plate XVIII, lig. 8 a and 6. 



Anisomyon centrale Meek, 1870, Geol. Surv. Wyoming & Contiguous Territories, 312. 



Among the collections made at Gallinas Creek, New Mexico, there is 

 a single internal cast in indurated clay, which I refer with comparatively 

 little doubt to A. centrale Meek. Its height is proportionally much less 

 than that of typical examples of this species. This imperfection, together 

 with the absence of the test, render it insufiicient of itself to base a specific 

 desci-iption upon ; consequently, I give the substance of Mr. Meek's original 

 description as follows : — 



"Shell depressed-conical; apex nearly central; slopes nearly straight 

 all around, but sometimes the anterior and sometimes the posterior slope is 

 a little convex ; marginal outline nearly circular or very broadly oval. 

 Siu'face marked by concentric lines of growth, by slightly-raised radiating 

 strise, and also by somewhat irregular radiating furrows of unequal dejith ; 

 the fuiTows being larger and deeper upon the anterior half of the shell. 



"Breadth of the largest specimens seen, 1.16 inches; height, about 

 0.95 inch." 



Our example is represented in figure 8 a and h, of natural size, by 



