1915] 



Clark: Fauna of the San Pablo Group 



461 



to a line dropped from the beak to the lowest point on the ventral 

 edge ; posterior dorsal edge long and gently arcuate ; posterior ex- 

 tremity evenly arcuate; ventral edge strongly arcuate. Surface of 

 shell marked by concentric incremental lines, which are medium fine 

 at the beaks, becoming coarser toward the base. Surface depressed 

 along the posterior dorsal margin into an escutcheon-like area. Hinge 

 plate heavy posteriorly. Right valve with four cardinal teeth; pos- 

 terior cardinal elongate, weak, on some specimens examined almost 

 obsolete and represented only by a roughened area ; second cardinal 

 elongate, deeply bifurcate, the posterior lobe being the longer ; third 

 cardinal heavy, short and slightly bifurcate ; anterior cardinal short 

 and thin. Left valve with three fairly heavy cardinal teeth and one 

 heavy, low, short, anterior lateral ; the posterior cardinal is more 

 elongate than the middle and anterior cardinals and is quite strongly 

 corrugated on the outer side. 



This species has been listed by previous writers as D. ponderosa 

 Gray, a Recent form now living in the Gulf of California and very 

 abundant in the Pleistocene of Southern California. The writer has 

 had the opportunity of examining a considerable number of specimens 

 of D. ponderosa Gray in the collections of the California Academy 

 of Sciences at San Francisco and of the University of California. D. 

 merriami, n. sp., differs from D. ponderosa Gray in that it is higher 

 in proportion to the width ; the posterior dorsal edge is more arcuate 

 and more oblique, the posterior extremity being more rounding; D. 

 ponderosa Gray lacks the slightly flattened surface along the posterior 

 dorsal edge, as seen on D. merriami, n. sp. ; also, on the latter species 

 the beaks are slightly more prominent and the lunule is not so deeply 

 impressed as on the former. The hinge plates of the two species are 

 quite different; the posterior cardinal on the left valve of D. mer- 

 riami, n. sp., is rather strongly corrugated, while on D. ponderosa 

 Gray it is not ; also, on the latter species the third cardinal of the right 

 valve is not bifurcated and is higher than on D. merriami, and the 

 anterior cardinal is less elongate, also the anterior lateral is more 

 approximate to the cardinal. 



Dosinia merriami, n. sp., somewhat resembles D. whitneyi Gabb, 

 which equals Chione whitneyi Gabb (Palaeontology of California, vol. 

 2, p. 23, pi. 5, fig. 40), which equals D. mathewsonii Gabb (Palaeon- 

 tology of California, vol. 2, p. 37, pi. 15, fig. 16), a species found in the 

 Lower Miocene of California. The hinge plate of D. whitneyi Gabb 

 is not known; the shell is usually not so large as I), merriami, n. sp., 



