1922] Trash: The Briones Formation of Middle California 145 



A large number of the species peculiar to the Briones and the San 

 Pablo are highly ornamented forms, or are of types which do not 

 have long ranges, and which, if the Briones were a distinct period, 

 probably would not extend into the San Pablo, such as the following : 



Astralium raymondi Clark 

 Cancellaria pabloensis Clark 

 Trophon gracilis clarki, n. nom. 

 Ficus stanfordensis Arnold 



Pecten raymondi Clark 



Pecton raymondi brionianus, n. var. 



Pecten bilineatus Clark 



In addition to these highly ornamented types, some forms are 

 relatively common in the Briones which as yet have not been found 

 in the San Pablo, but which are very closely related to some San Pablo 

 species. These are : 



Briones Form 

 Astrodapsis brewerianus diablo- 



ensis Kew 

 Modiolus gabbi subconvexus, n. var. 

 Trophon daviesi, n. sp. 



San Pablo Homologue 

 ( Astrodapsis cierboensis Kew 

 | Astrodapsis tumidus Eemond 

 Modiolus gabbi Clark 

 Trophon ponderosum Gabb 



The general character or facies of the fauna of the Briones and 

 the San Pablo appears to be very similar. A large number of species 

 are numerically quite abundant in both formations : 



Area trilineata Conrad 

 Diplodonta parilis (Conrad) 

 Dosinia arnoldi Clark 

 Dosinia merriami Clark 

 Dosinia merriami occidentalis Clark 

 Metis alta Conrad 

 Mulinia pabloensis Packard 

 Mya oralis (Conrad) 

 Ostrea bourgeoisi Eemond 

 Pecten raymondi brionianus n. var. 

 (San Pablo homologue, P. ray- 

 mondi Clark) 



Saxidomus nuttalli Conrad 

 Sehizothaerus nuttallii (Conrad) 

 Siliqua lucida (Conrad) 

 Solen perrini Clark 

 Solen sicarius Gould 

 Spisula albaria (Conrad) 

 Spisula catilliformis Conrad 

 Calyptraea filosa (Gabb) 

 Crepidula praerupta Conrad 

 Natica kirkensis Clark 

 Natica pabloensis Clark 



The above fossils are very common and constitute numerically over 

 one-half of the specimens found in the Briones, yet they are also cpiite 

 abundant in the San Pablo. 



The lithologic sequence of the Briones and the San Pablo is very 

 similar. Both possess massive, hard, fossiliferous sandstones near the 

 base of the formation. The upper parts of both consist of alternating 

 fine-grained sandstones and sandy shales. As yet, no differences in dip 

 and strike have been observed between the Briones and the San Pablo. 



Hence from the faunal similarity to the San Pablo, and the faunal 

 distinctness from the Monterey, it is concluded that the Briones 



