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University of California Publications in Geology [Vol. 9 



The geographic range of the Recent species and the vertical range 

 of the fossil species are shown on the accompanying chart, in which 

 the periods are divided into stages according to the faunas. 



PHYLOGENY OP THE PACIFIC COAST MACTRINAE 



The genus Spisula is the first genus of the Mactrinae to appear 

 within this Pacific Coast region. It is doubtfully represented in the 

 Horsetown Beds by S. ashburnerii (Gabb), the typical species of 

 the subgenus C ymbophora. Higher in the Cretaceous, this form is 

 associated with 8. gabbiana (P. M. Anderson) and *S'. chicoensis, 

 new name. These three are characterized by having a spoon-shaped 

 chondophore with raised margins and a type of right cardinal in 

 which the anterior arm is lacking. This typical Cretaceous group 

 is replaced in the Eocene by the subgenus Hemimactra in which the 

 chondophore is submerged below the level of the hinge plate and in 

 which the right cardinal bears two well-developed arms. By the 

 middle of the Miocene this subgenus was the dominant one of the 

 family, including the great majority of the Tertiary species, many of 

 which had a wide geographic range. The genus Spisula reached its 

 maximum during the late Miocene, declining in importance until at 

 present it is represented by but one more species in this Coast region 

 than is the younger genus Mactra. 



The first undoubted mactroid species appears in the San Pablo. 

 It is not until the Pleistocene that this genus holds an important 

 place in the fauna of the region, being represented at that time by 

 two groups of species. These two subgenera, Mactrotoma and Mac- 

 trella, are quite widely separated groups. The latter appeared earlier 

 on this Coast, being represented by the species Mactra trampasensis, 

 n. sp. To the former belong the greater number of Recent species. 

 The genus Mactra appears at present to be at its maximum of de- 

 velopment. 



The phylogenetic history of the genus Mulinia on the West 

 Coast of the United States is characterized by its sudden appear- 

 ance, rapid dispersal and abrupt disappearance. It first appears 

 in the early Miocene. By the time the San Pablo stage was reached, 

 this genus had become differentiated into several types, that were 

 distributed over the coast from Washington to southern California. 



