1915] 



Clark: Fauna of the San Pablo Group 



499 



are narrower than the ribs. On the body whorl there are about twenty 

 to twenty-one of these ribs. On the whorls of the spire there are only 

 three or four. Outer lip thin, smooth internally ; inner lip incrusted. 

 Peristome elongate-ovate. Canal open, straight, fairly long. Umbilicus 

 subperforate. 



Named in honor of Dr. R. E. Dickerson, assistant curator of 

 palaeontology of the California Academy of Sciences, San Francisco. 



Dimensions. — Type specimen: height, 20 mm.; greatest diameter 

 of body whorl. 11 mm. ; height of body whorl, 15 mm. 



Occurrence. — Lower San Pablo on San Pablo Bay, University of 

 California locality 410. 



TROPHON LAWSONI, n. sp. 

 Plate 67, figures 10 and 11 



Shell large, robust, thick; spire moderate in height, with four to 

 five whorls, the last two or three being small ; apex acute ; body whorl 

 large. Surface of whorls sculptured by about eight or nine promi- 

 nent, thick, lamella-like varices; on some specimens the varices are 

 angulated but never spinose ; whorls angulated, with the surface slop- 

 ing up gently from the angle to the suture ; sutures obscurely ap- 

 pressed; no revolving ribbing apparent. Outer lip thick; inner lip 

 smooth. Canal short, narrow, deep and strongly recurved. 



Named in honor of Professor A. C. Lawson, Professor of Geology 

 at the University of California. 



Dimensions. — Type specimen: maximum altitude, about 93 mm.; 

 maximum width of body whorl, 64 mm. 



Occurrence. — In the Upper San Pablo Group on San Pablo Bay, 

 University of California locality 1611, and on the south side of Mount 

 Diablo, University of California locality 1499. 



TROPHON PONDEROSUM Gabb 

 Plate 66, figures 3 and 4 

 Trophon 'ponderosum Gabb, Palaeontology of California, vol. 2, p. 2, pi. 

 1, fig. 3. 



Shell moderately robust, thick; spire subelevated; whorls five; angulated in 

 the middle, sloping above; suture irregularly impressed; surface marked by 

 eight or nine blunt varices, prominent and acute on the angle of the whorl, 

 and commonly becoming more or less obsolete above and below; in the upper 

 whorls the varices are replaced by mere elevated ribs, which become smaller 

 until in the first three volutions they barely exist as undulations. Aperture 

 broad; canal open, short, slightly recurved; columelar lip heavily incrusted; 

 outer lip entire, subacute. Umbilicus well marked, but closed. 



