352 MEXICAN BO UNDARY SHELLS— BALL. 



species. In northern IMexico, Sonora offers 2, Cliiliuabua 1, and Coa- 

 binla 1, making 4 for this area; while on the United States side of the 

 boundiiry, Texas west of the Colorado Kiver affords 3, New Mexico 3, 

 and Arizona 1 si)ecies or 7 in all, of which 3 described in this paper 

 are due to the collections made on the IJouudary Commission by Dr. 

 Mearns. A large number of species i)robably remain to be made known. 



Genus COELOCENTRU M, Crosse and Fischer. 



COELOCENTRUM NELSONI, new species. 



(Plato XXXIII, fij^s. 5, 6.) 



Shell, large, strong, decollate, retaining *.» whorls subcylindric, atten- 

 uated more rajjidly at the upper 2 or 3 whorls, periphery tlattish, 

 suture distinct, strong, not deep; transverse sculpture of numerous 

 rather asperate coucavely arched little raised ridges stronger near the 

 sutures and usually with wider interspaces; spiral sculpture of some- 

 what vermicular obscure charactc^r like the nuirkings left by a " sand- 

 blast,-' but occasionally developing sparse distant tine spiral riblets, 

 and stronger on the later whorls; base rounded except for the obscure 

 peripheral line, the nnibilicus almost closed; aperture rounded below, 

 slightly angular, oblicpie, free from the body whorl, with the margin 

 continuous and reflected but narrow; axis normal, nearly closed at 

 the decollation where it is small but large in the later whorls; color 

 pale straw. Length 53, maxinuim diameter 18, diameter at decolla- 

 tion 8 mm. 



Type fnun Tuxtla, Mexico; collected by E. W. Xelson, of the United 

 States Agricultural Department. No. 1073G8, U.S.N.M. 



This shell recalls Eucalodium compactum, Pilsbry, but is more cylin- 

 drical and stouter, besides having a totally different axis. It is the 

 largest known species of the genus, and is dedicated to the indefatiga- 

 ble naturalist, E. W. Nelson, who has added so largely to our knowledge 

 of the American fauna from Alaska to Guatemala. 



COELOCENTRUM PFEFFERI, new species. 

 t;Plate XXXIII, ligs. 1,2.) 



Shell subcylindric, with 8^ whorls, attenuated above, rounded below, 

 solid, decollate, the whorls gently rounded with a distinct suture, 

 Avithout spiral sculpture, transverse sculpture of delicate, hardly arched, 

 little raised, crowded lines, subequal over the whole surface; base 

 rounded, the basal area bounded by an obscure line, umbilicus redu(;ed 

 to a minute perforation, aperture rounded below, slightly angular above, 

 usually free but occasional adnata to the body whorl; axis normal, 

 small at the decollation: <;olor pale livid ])ink, whitish near the aper- 

 ture. Length 43, maximum diameter lo, diameter at the decollation 

 7 mm. No. 107367, U.S.N.M. 



