ARNOLD — THE PALEONTOLOGY AND STKATIGKAPHY OP SAN PEDRO. 273 



the base; last whorl slightly angular at the periphery, base of this whorl smooth except for very fine 

 axial, incremental lines, and nearly obsolete spiral striations; aperture subquadrate, the thick outer 

 lip joins the nearly straight, somewhat revolute columella at almost a right angle. 

 Dimensions. — Long. 6 mm.; lat. 1.8 mm.; body-whorl, altitude 2 mm. 



This species somewhat resembles T. tridentata in general appearance, but may 

 be di.stinguished from that species by the greater number of ribs, which are closer 

 together and broader, the narrower interspaces and fewer whorls. The si^ecimen 

 described lacks the nuclear whorls, which were broken off; the spiral sculpture on 

 this specimen is also very faint, and would have been overlooked if a very high- 

 power magnifier had not been used. This specimen was identified by Dr. Dall; but 

 the species was omitted from the text prepared by Dall and Bartsch. 



Rare in the lower San Pedro series at Deadman Island and San Pedro; and 

 in the upper San Pedro series at Los Cerritos and San Pedro. 



Living. — Puget Sound to Santa Barbara (Carpenter) : San Pedro (Williamson). 



Pleistocene. — San Pedro (Arnold). 



290. Turbonilla (Lancea) tridentata Carpenter. 



Pi^TE II, Figs. 1 and la. 



Chemnitzia tride7itala Cpr., Jour, de Conch., Vol. XIII (3rd Ser., Vol. V), 1S65, p. 147. 



Shell large, broad; chestnut colored, obscurely banded; nuclear whorls three, helicoid, 

 about one-third immersed, scarcely extending beyond the margin of the spire, their axis being at a 

 right angle to the axis of the later whorls; post-nuclear whorls slightly convex, somewhat contracted 

 below and slightly shouldered at the summit; traversed by about twenty to twenty-four strong, well- 

 rounded, somewhat oblique axial ribs, which continue faintly over the decidedly angular periphery 

 of the last whorl and the base to the umbilical region; these ribs are considerably enfeebled on the 

 last whorl of old shells and frequently become almost obsolete on these; the exposed portion of the 

 whorls is traversed by six to ten spiral grooves, which appear most prominently in the shallow and 

 broad intercostal spaces, and less so on the ribs; these deep spiral lines are regularly spaced, leaving 

 a broader interval on the middle of the exposed portion of the whorl; the base of the last whorl is 

 likewise ornamented by spiral grooves, but here they appear less developed than on the spire; in 

 addition to this the entire surface of the shell is marked by numerous very fine, somewhat wavy, 

 spiral and axial striae, which show most prominently on the last whorl and base, and give the shell a 

 very minutely reticulated secondary sculpture; at irregular intervals the whorls are marked by thick 

 callous varices, which are usually of a lighter color than the remainder of the shell; aperture large, 

 subquadrate; posterior angle acute; outer lip thin, having three strong internal lirations, joining the 

 whitish, short, straight, revolute columella at a little less than a right angle; by transmitted light 

 two spiral, light color-bands become apparent on the inside of the lip, each of which is bordered by 

 a zone of a darker color than the remaining shell; the general color effect of the exterior is that of a 

 flesh-colored shell, covered by a dark epidermis, which is stretched tight over the ribs, permitting 

 the lighter color beneath to shine through it at their summits. 



Dimensions. — Long. 11. i mm.; diam. 3.2mm. (Dr. Carpenter's type, collected at Monterey, 

 and having 11 post-nuclear whorls). Long. 12.8 mm.; diam. 3.6 mm. (Specimen figured, which is 

 from San Pedro). 



Fossil specimens from the post-Pliocene of California are larger, have fewer 

 ribs, and more of the deep revolving lines than recent ones. 



(35) March 30, 1903. 



