456 University of California Publications in Geology [Vol. 8 



M. expansus Arnold, described by Arnold as a variety of M. 

 mathewsonii Gabb, is somewhat similar in outline to M. merriami, n. sp., 

 but differs in being- less convex; the posterior slope is less steep and 

 the posterior margin is not angulated as is the case of M. merriami, 

 n. sp. ; the latter also lacks the radial striations seen on M. expansus. 



It would seem to the writer that Arnold's M. mathewsonii var. 

 expansus should be classed as a true species, not a variety. M. expansus 

 Arnold belongs to the section Hormomya Moreh according to Dall's 

 classification. M. mathewsonii Gabb belongs to the section Mytilus. It 

 does not appear to be radially striated as stated by Dall in his dis- 

 cussion of the species of Mytilus of the Pacific Coast (Transactions of 

 the Wagner Free Institute of Science, vol. 3, pt. 4, p. 789, 1898). 



Dimensions. — Greatest antero-posterior diameter, about 104 mm.; 

 greatest width as measured at right angles to the base, about 55 mm. ; 

 height of the depressed anterior margin, about 42 mm. 



Occurrence. — From the upper beds of the Upper San Pablo Group 

 on the south side of Mount Diablo, University of California locality 

 118. 



Named in honor of Professor J. C. Merriam, Professor of Palae- 

 ontology at the University of California. 



MYTILUS PERRINT, n. sp. 

 Plate 42, figures 5 and 6 

 Shell medium in size, somewhat variable in outline, elongate sub- 

 ovate ; beaks subacute, slightly twisted ; base straight or slightly con- 

 cave. Posterior cardinal angle as a rule slightly anterior to the middle 

 of the elongate axis, the angle being obscured on some of the larger 

 specimens, the margin posterior to the point of angulation being 

 nearly straight and parallel or slightly diverging from the anterior 

 edge. Ventral edge broadly rounded to subtruncate. Surface of 

 shell covered by heavy irregular undulations with finer incremental 

 lines. Anterior slope of surface fairly abrupt; posterior slope gentle. 

 There is a tendency for the surface of the shell to be depressed and 

 flattened just in front of the posterior dorsal edge. Hinge plate 

 heavy below the beak; on some specimens the thickened hinge plate 

 resembles somewhat the myaphoric septum of a Septifer or Dreissensia ; 

 the anterior edge of the smaller specimens is denticulate immediately 

 below the beaks; on the smaller specimen figured at least ten of these 



