Clark: The San Lorenzo Series of Middle California 135 



Genus MYTILUS Linn6 



MYTILUS ARNOLDI, n. sp. 

 Plate 12, figure 1 



Type specimen 11153, Coll. Invert. Palae. Univ. Calif., loe. 78 

 Shell medium in size; beaks subacute; posterior end rather narrow, somewhat 

 acurely rounded; base straight or nearly so; posterior dorsal margin straight, 

 extending not quite to the middle of the shell ; posterior to this, the margin con- 

 verges very strongly to the posterior end, the converging margin being gently 

 convex ; the change from the posterior dorsal margin to the converging posterior 

 margin is not marked by an angulation but by a broad, regular curve. Convexity 

 of surface rather broad, the posterior slope being gentle, with the anterior slope 

 slanting more steeply though being considerable from vertical. Surface smooth 

 except for medium coarse, somewhat irregular lines of growth. 



Dimensions. — Type specimen: maximum length, 63.5 mm.; maximum width, 

 33 mm. 



Occurrence. — Type from near the base of the Kirker formation near Kirker 

 Creek, University of California locality 78. Localities 1131 and 1309 in the 

 San Ramon formation. 



The type of this species came from the north side of Mount Diablo, 

 where it was associated with Aeila shumardi Dall. It is also found 

 in the Agasoma gravidum beds to the west of Mount Diablo. The 

 specimens in these latter beds are smaller than those found to the 

 north of Mount Diablo, but they seem to be referable to the same 

 species. 



MYTILUS (MYTILOCONCHA) MATHEWSONII Gabb 

 Plate 18, figures 1 and 2 



Mytilus mathewsonii Gabb, Geol. Surv. Calif., Palaeontology, vol. 2, sec. 1, 



p. 30, pi. 8, fig. 51; 1866. 

 Mytilus mathewsonii Gabb, Merriam, Univ. Calif. Publ., Bull. Dept. Geol., 



vol. 3, no. 16, p. 378, 1004. 

 Mytilus mathewsoni Gabb, Dall, Trans. Wagner Free Inst. Sci., vol. 3, 



part 4, p. 789, 1898. 

 Mytilus mathewsonii Gabb, Smith, Proc. Calif. Acad. Sci., ser. 4, vol. 3, 



p. 172, 1912. 



Mytilus mathewsonii Gabb, Clark, Univ. Calif. Publ., Bull. Dept. Geol., 

 vol. 9, no. 2, p. 14, 1915. 



M. mathewsonii is distinguished by its comparatively large size, smooth sur- 

 face, blunt beaks and straight base. The shell is noticeably high, rather narrow; 

 the slopes from the line of highest convexity are steep both anteriorly and poster- 

 iorly. It belongs to the subgenus Mytiloconcha Conrad; the hinge plate just 

 under the beak is thick and on this thickened area is a broad, deep, V-shaped 

 groove. The ligamental groove is long, fairly deep and heavy. 



Occurrence. — A common species in the basal Agasoma gravidum beds, Univer- 

 sity of California localities 1131, 14, 1309, etc. 



