1922] Trash: The Briones Formation of Middle California 149 



An analysis of over one hundred specimens of the forms of Pecten 

 raymondi Clark found in the Briones and the San Pablo formations 

 shows that there are two end varieties with a gradual gradation between. 

 The San Pablo forms, as a general rule, possess a relatively more 

 convex left valve, and higher and stronger ribs which are relatively 

 close together. This is the typical P. raymondi described by Clark. 19 

 The forms found in the Briones have only slightly convex valves, and 

 the ribs are relatively low and widely separated. The name P. ray- 

 mondi brionianus is suggested for this variety. 



PECTEN (PECTEN) ANDERSONI GONICOSTUS, n. var. 

 Plate 1, figure 5 

 Type— No. 12370, Univ. Calif. Mus. Pal. 



Shell small to medium in size, subcireular, nearly equilateral; dorsal margins 

 gently concave; anterior dorsal margin longer than posterior; ventral margins 

 strongly arcuate; right valve gently convex, and possesses about seventeen sub- 

 angular "V" shaped ribs; interspaces wider than ribs; hinge line about three- 

 fifths width of shell; ears about equal in length; anterior ear with five ribs; 

 posterior ear with four fine subangulate ridges, with interspaces wider than 

 the ridges; byssal notch prominent. 



Dimensions. — Type specimen U. C. no. 12370; alt., 29.7 mm.; width, 30.2 mm.; 

 length of ears, 18 mm.; apical angle, 100°. 



Occurrence. — Briones formation, U. C. loe. no. 1176. 



This species resembles P. andersoni Arnold, 20 but it differs from 

 the latter in that the right valve is less convex, the ribs are less promi- 

 nent, the interspaces are wider, and the ears are shorter and broaden 



Family Mytilidae 

 Genus MODIOLUS Lamarck 

 MODIOLUS GABBI SUBCONVEXUS, n. var. 

 Plate 3, figure 2 

 Type.— No. 12372, Univ. Calif. Mus. Pal. 



Shell very similar to M. gabbi Clark, 21 but differs from the latter in that the 

 umbonal ridge is less prominent; the shell is more convex, narrow, and tumid; 

 the striations are slightly narrower and less prominent; and on the posterior 

 slope the striations become much finer, closer together and more numerous. 



Dimensions. — Type specimen U. C. no. 12372; alt., 56.8 mm.; greatest width, 

 20.6 mm. 



Occurrence. — This is a common species in the Briones formation. Type from 

 U. C. loe. no. 793. 



10 Clark, B. L., Univ. Calif. Publ. Bull. Dept. Geol., vol. 8, p. 450, pi. 46, figs. 

 1 and 2, pi. 47, figs. 1 and 2, 1915. 



20 Arnold, U. S. G. S. Prof. Paper 47, p. 82, pi. XXIV, figs. 5-8, 1906. 



21 Clark, Univ. Calif. Publ. Bull. Dept. Geol., vol. 8, p. 458, pi. 48, fig. 1, 1915. 



