488 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Of the intermediate valves examined from the same material, 



2 show 12 longitudinal riblets on each side of the central areas, 



3 show 11 (or 10-11), 3 show 10 (or 9-10), 3 show 9 (or 8-9), 

 2 show 8 (or 7-8), and 1 shows 7. 



On the tail valves the riblets of the central areas are fewer : 

 6 on a side (or 5-1-5) in 6, 7-1-7 in 1, 8-1-8 in 1, 6-1-6 in 1, 

 and 5 on a side in 1 specimen. 8 of these same valves show 

 5 strong radial ribs each, the other 3 shells having 6 each. 2 

 have 18 slits, 4 have 16, 3 have 15, and 1 has 14. No relation 

 is evident between a maximum number of ribs and an in- 

 creased quota of slits. Toward the base the ribs become double, 

 and in larger shells triple, quadruple, or (in the case of the 

 outermost pair) even quintuple at the base in similar fashion 

 to the radial ribs of the other valves. 



The specimens from the Lower San Pedro Formation at 

 Nob Hill run rather larger than those from Santa Monica, 

 and have unusually sharp, clear sculpture and teeth. Among 

 these have been noted 7 head valves with the typical slit 

 formula 1-1-2-1-2-1-1, none being certainly atypical. Of the 

 median valves, 5 show longitudinal riblets on the central areas 

 as follows: 11-11, 11-11, 11-10, 11-10, 5-5, the last a very 

 small specimen. Three tail valves show 14 slits each, radial 

 ribs respectively 7, 6, 5, and longitudinal riblets on central 

 areas 8-9, 6-7, 4-4. 



The teeth of Callistochiton, as evidenced by the present 

 species, seem to be composed of a somewhat different substance 

 from the more spongy deposit which fills in the slits, so even 

 when quite worn down it is quite possible to distinguish their 

 number and shape (in transection) with perfect accuracy, 

 especially if the surfaces in question are slightly moistened, 

 when the details are brought out with great clearness. This 

 is fortunate as in most fossil specimens the teeth show as little 

 more than a pattern, scarcely worthy of the name teeth at all. 



Caliper measurements of some of the larger specimens are 

 as follows : 



