458 University of California Publications in Geology [Vol. 8 



anterior slope steep ; surface of shell posterior to the steep anterior 

 slope slightly convex, with a very gentle posterior slope. Surface 

 smooth except for fine incremental lines of growth. Hinge plate 

 below the beaks heavy, with a deep groove anterior to the ligamental 

 groove. 



This species differs from Mytilus coalingensis Arnold in being 

 more slender, not so broad posteriorly ; the beaks are not twisted ; the 

 surface is not so high or convex, the anterior slope not being so high 

 but possibly steeper. 



Occurrence. — In the Upper San Pablo Group on Las Trampas 

 Ridge and Rocky Ridge near southeast corner of Concord Sheet, Uni- 

 versity of California localities 38, 39, 1237. 



MODIOLUS GABBI, n. sp. 

 Plate 48, figure 1 



Modiolus multiradiatus Gabb (in part), Pal. Calif., vol. 2, p. 30, not 

 pi. 8. 



Modiolus multiradiatus Gabb (Smith), Proc. Calif. Acad. Sci., 4th ser., 

 vol. Ill, p. 172. 



Shell medium to large, elongate, anterior end extending only 

 slightly beyond the beak. Posterior dorsal edge subangulate posterior 

 to the mid-length of the valve. Base nearly straight with a tendency 

 to concavity ; posterior end evenly rounded. Surface sculptured by 

 rather broad, flat-topped, dichotomous ribbing, except for an elongate, 

 smooth, subtriangular space above the base near the anterior end; 

 the dichotomous ribbing is more marked toward the posterior margin, 

 while anteriorly the ribs are split only at their lower ends ; interspaces 

 slightly narrower than the tops of the ribs. Surface quite strongly 

 convex, usually with a well-defined ridge marking the line of greater 

 convexity extending from the beak toward the basal side of the pos- 

 terior extremity ; on the larger specimens this ridge is not so pro- 

 nounced, the surface being more evenly rounded. 



This species has heretofore been known as Modiolus multiradiatus 

 Gabb, which Gabb reported from two localities, San Emideo ranch, 

 west of Fort Tejon, and the Miocene of Contra Costa County. The 

 type specimen without doubt came from San Emideo ranch. Some of 

 Gabb's original material from this locality is in the collections of the 

 Department of Palaeontology at the University of California. The 

 form from San Emideo ranch differs from the one in the Upper Mio- 



