474 University of California Publications in Geology [Vol.8 



slightly flexed and with posterior margin more strongly depressed 

 than on left. Ligamental groove less than half the length of posterior 

 dorsal edge. Surface of valves covered by fine incremental lines and 

 occasional lines of interrupted growth. 



M. andersoni is of the same general type as M. indentata Carpenter, 

 a Recent species on the West Coast; it differs in that the ventral edge 

 is not so arcuate, and does not have the distinct concavity seen on the 

 latter along the line of flexure ; otherwise the two forms are very close 

 and possibly the one should be called a variety of the other. This 

 difference, however, appears to be quite constant; therefore the writer 

 has given the San Pablo form specific rank. 



M. andersoni is found in the same general horizon as M. diabloensis, 

 n. sp. It differs from that species in the following respects: it is 

 longer in proportion to the height ; the right valve is not so strongly 

 flexed nor so strongly depressed along the posterior dorsal margin. 

 M. diabloensis lacks the distinct flange along the posterior dorsal 

 margin, seen on M. andersoni. In this respect the former is more 

 closely related to M. nasuta Conrad and M. inquinata Deshayes, while 

 the latter resembles M. secta Conrad and M. indentata Carpenter. 

 Again, the ligamental groove is proportionately longer on M. diablo- 

 ensis than on M. andersoni. 



M. andersoni somewhat resembles M. vanvlecki Arnold from the 

 Etehegoin, which is also of the same general type as M. indentata. It 

 differs, however, in that the depressed posterior margin is narrower 

 and is not sharply defined by a carina as is the case with M. vanvlecki; 

 the posterior flexure of the right valve is not so great ; the ventral edge 

 is straighter, lacking the strong concavity along the line of flexure 

 seen on M. vanvlecki. This last species appears to be more closely 

 related to M. indentata than does M. andersoni. 



Named in honor of Mr. F. M. Anderson, curator of Palaeontology in 

 the Museum of the California Academy of Sciences, San Francisco. 



Dimensions. — Length, 31 mm. ; height, 20 mm. ; diameter, about 

 5 mm. 



Occurrence. — This is a fairly common species in the Upper San 

 Pablo Group. It has been found in the San Pablo Bay section, on 

 the south side of Mount Diablo and in the Tice Valley Syncline. Uni- 

 versity of California localities 1470 (type), 1227, 749, etc. 



