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



mucro; sculptured by about 25 low, radiating ribs, obsolete 

 toward the mucro. Central area sculptured in similar fashion 

 to the corresponding region on the median valves, the longi- 

 tudinal riblets about 15 on a side. Interior with a triangular, 

 strongly dendritic callus, its central portion showing about 4 

 irregular, longitudinal rows of the transverse pores leading 

 into the short, weakly toothed plate connecting the sutural 

 laminae across the shallow sinus. Teeth similar to those of 

 head valve, but much shorter, and fitting in under the eaves 

 more closely. Slits 10, each the marginal terminus of the usual 

 radiating series of transverse, slit-like pores. 



Caliper measurements of the type and paratypes are as 

 follows : 



Head valve Med. valve Med. valve Tail valve 

 Type 

 [881] [883] [884] [886] 



Length 3.5 mm. 2.8 mm. 2.6 mm. 2.7 mm. 



Diameter 6.8 7.7 7.7 4.7 



Height 2.0 3.4 3.5 1.5 



Remarks: In the case of this species comparison is chiefly 

 needed with /. pectinulatus, I. siuudentatus, and perhaps /. 

 dceipiens, although in the last instance the insufficiency of the 

 original description, as well as the rarity of authentic specimens, 

 precludes any positive opinion. From both the other species /. 

 sanctcemoniece differs in the much more numerous longitudinal 

 riblets of the central areas. From /. pectinulatus it further 

 differs in its much smaller size, the lack of prominent pustules 

 on the ribs of the lateral and terminal areas, and the altogether 

 more delicate scheme of sculpturing. 



It is a very pretty little species occurring not rarely in the 

 type horizon. 



31. Ischnochiton (Lepidozona), species 



A nearly perfect anterior valve collected in the Upper San 

 Pedro Pleistocene of Los Cerritos Hill, Long Beach, California, 

 by T. S. Oldroyd [664], does not seem to be referable to any 

 of the foregoing or other described species. It is apparently 

 quite close to /. sanetcemonicce, but differs from it in its greater 

 elevation, steeper front slope, much narrower ribs, and pecu- 



