1916] 



Packard: Mesozoic and Cenozoic Mactrinae 



275 



Conrad, Mulinia densata Conrad and M. pabloensis, n. sp. Only 

 the first of these is restricted to this portion of the series, its highest 

 known range being the base of the Astrodapsis whitneyi zone of the 

 middle San Pablo. To this zone belongs the earliest undoubted 

 mactroid species, M. trampasensis, n. sp. To the upper San Pablo 

 belong the following forms: Spisula albaria (Conrad), 8. catilli- 

 formis Conrad, Mulinia densata (Conrad) and M. pabloensis, n. sp. 

 The last-mentioned is restricted in its upward range to the top of the 

 San Pablo. 



The exact stratigraphic position of the following Miocene species 

 has not been definitely determined with reference to the standard 

 California section: Mulinia landesi (Weaver), M. alta (Weaver) and 

 M. undilifera (Weaver). 



The following species range through the Jacalitos into the 

 Etchegoin formation: Spisula albaria (Conrad), 8. catilliformis 

 Conrad and Mulinia densata Conrad. 8. coaling ensis (Arnold) is 

 known only from the Etchegoin. Two other species, S. voyi (Gabb) 

 and S. brevirostrata, n. sp., occur at a questionably equivalent strati- 

 graphic position in the Wildcat series of Humboldt County, 

 California. 



The Merced fauna of the San Francisco Peninsula has yielded the 

 following mactrine species: Spisula albaria (Conrad), 8. catilli- 

 formis Conrad, S. mercedensis, n. sp., and 8. falcata (Gould). Two 

 of these, 8. albaria (Conrad) and S. mercedensis, n. sp., became 

 extinct at the close of the Pliocene. 



The San Pedro or Pleistocene fauna is characterized by the 

 appearance of two mactroid species of widely different types. The 

 known Pleistocene species include : Mactra calif ornica Conrad, 

 M. exoleta Gray, Spisula catilliformis Conrad, 8. falcata (Gould) 

 and 8. hemphillii (Dall). 8. voyi (Gabb) may be mentioned here, 

 since it is known to occur both in the Pliocene and Recent faunas. 

 Its absence from the San Pedro fauna may be due to the fact that 

 it is a northern species, being at present limited in its southward 

 range by the Straits of Juan de Fuca. 



Three species, Mactra dolabrif ormis (Conrad), M. nasuta Gould, 

 and Spisula planulata (Conrad) are known definitely only in the 

 Recent fauna. To these should be added the above-mentioned 

 Pleistocene forms, giving a list of four mactroid and five spisuloid 

 species known to the waters of the Pacific Coast north of Mexico. 



