12 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.61 



phonal bands are broad and shallow and are separated by a ribbed 

 band about 20 mm. wide at the base and broadening to about 35 mm. 

 at the top. The anterior siphonal area (E) forms a shallow channel 

 broadening from a width of 17 mm. at the base to about 25 mm. at the 

 top and is ornamented by 12 or more low, rounded, rather indistinct, 

 irregular ribs. The posterior siphonal channel (S) is shallower, 

 flatter, and a little narrower than the anterior one and is ornamented 

 by 9 or 10 similar small ribs. The band between the siphonal areas 

 exhibits two bordering rather prominent, though poorly preserved, 

 ribs, separated by a deep, broadly flaring, V-shaped channel, on the 

 front side of which is one subordinate rib and on both sides of which 

 are fine indistinct longitudinal lines. 



The rest of the surface of the shell is marked by about 12 unequal, 

 angular, more or less prominent ribs, separated by rounded to angular 

 channels, the sides of most of which are ornamented with several in- 

 distinct minor longitudinal ribs. The major ribs are largest on that 

 part of the surface behind the posterior siphonal area and smallest 

 on the half of the surface which lies in front of the anterior siphonal 

 area. 



Locality. — From gullies, known as El Colorado, on Chocoy ha- 

 cienda, 1^ kilometers northwest of Omaha station, near the north- 

 east corner of fraction 101, State of Tamaulipas, Mexico. Collected 

 by Messrs. A. E. Fath and Eugene Stebinger. 



Type.—Doii'AiQd to the United States National Museum by the 

 collectors, A. E. Fath and Eugene Stebinger. Cat. No. 32505, 

 U.S.N.M. 



Geologic position.— Uipiper part of Mendez shale, which is an Upper 

 Cretaceous formation corresponding in age approximately to the 

 Taylor marl of Texas. 



DURANIA MANUELENSIS, new species. 



Plate 15. 



Description. — This species is based upon one relatively small, in- 

 complete lower valve which is perhaps a young individual. Origi- 

 nally the shell was doubtless elongate, conical, and roughly circular 

 in cross section ; as preserved, however, it presents an elongated oval 

 cross section due to mechanical compression. Only the outer shell 

 layer is preserved, and the internal cavity left by the removal of 

 the inner shell layer is filled with a calcareous shale matrix. 



The cell structure is rather coarse, and the cells are polygonal and 

 in cross section show only a slight tendency to form concentric rows ; 

 this is perhaps due to the fact that the growth layers were added 

 almost horizontally one upon the other. The upper rim of the shell 

 is nearly flat, though very slightly undulating, and bears faint, 

 widely spaced impressions of radial vessels. A low, narrow carina 



