134 University of California Publications in Geology [Vol. 11 



smooth, triangular space between the base and the umbonal ridge, sculptured by 

 numerous fine radiating diehotomous ribs, with interspaces averaging less than 

 the width of the ribs. The diehotomous character of the ribbing is more marked 

 toward the posterior margin. 



Dimensions. — Length, 38 mm.; greatest width, 17 mm.; semidiameter, about 

 6 mm. 



Occurrence. — University of California locality 2038. 



This species belongs to the same general type as Modiolus multi- 

 radiatus Gabb 110 and Modiolus gabbi B. L. Clark. The type of 

 M. multiradiatus Gabb came from San Emigclio ranch, west of Fort 

 Tejon, and is either Lower Miocene or Oligocene. Gabb's original 

 description included two species, the type and a species common in 

 the Upper Miocene of Middle California, Modiolus gabbi B. L. 

 Clark. 111 M. multiradiatus differs quite decidedly from M. kirker- 

 ensis, both in outline and sculpturing; the anterior end is not pro- 

 duced as strongly as that of M. kirkerensis and there is no well-marked 

 posterior angle ; also the sculpturing is much coarser. 



MODIOLUS PITTSBUEGENSIS, n. sp. 

 Plate 9, figure 9 



Type specimen 11122, Coll. Invert. Palae. Univ. Calif., loc. 1895 

 Shell rather small, thin; umbones not prominent; umbonal ridge indistinct, 

 unseen in the anterior third of the shell; anterior end strongly produced beyond 

 the beaks ; base straight ; posterior margin long, with no distinct posterior angu- 

 lation; posterior end broad and regularly rounded, the base and the posterior 

 margin diverging in approaching the posterior end. Surface rather strongly 

 arched, the highest point of convexity being posterior to the middle of the shell. 

 Anterior slope broad and fairly steep; posterior slope shorter but more steep 

 than the anterior. Surface smooth except for medium fine incremental lines. 



Dimensions. — Type specimen: length, about 32 mm.; maximum width measured 

 at right angles to base, 18.5 mm.; diameter of both valves, about 12 mm. 



Occurrence. — Type specimen from University of California locality 1895, in 

 the Kirker formation near the top of the Astoria series to the north of Mount 

 Diablo. 



This species appears to be quite distinct in outline from any of 

 the known Recent or fossil species of the "West Coast. In outline it 

 somewhat resembles Modiolus in flatus Dall; 112 both have the broadly 

 rounded posterior end ; on the former the anterior end is more 

 strongiy produced and the shell is more slender just back of the beaks. 



no For original description of Modiolus multiradiatus, see Geol. Surv. Calif., 

 Palaeontology, vol. 2, p. 30, pi. 8, fig. 52, 1868. 



in For original description of Modiolus gabbi, see Univ. Calif. Publ., Bull. 

 Dept. Geol., vol. 8, no. 22, p. 458, pi. 48, fig. 1, 1915. 



112 For original description of Modiolus inflatus, see Dall, W. H., U. S. Geol. 

 Surv., Prof, paper no. 59, p. 114, pi. 12, figs. 8 and 9, 1909. 



