160 University of California Publications in Geology [Vol. 11 



scars. The shell is trigonal in outline, only slightly longer than high ; 

 the beaks are fairly prominent and situated anterior to the middle of 

 the shell. The dorsal slopes are steep. There is a very distinct ridge, 

 extending from the beak to the posterior end and parallel with the 

 posterior dorsal margin, the narrow surface between this ridge and 

 the dorsal margin being depressed almost at right angles to the main 

 surface of the shell. Another specimen of a Mulinia? was found at 

 University of California locality 1309 ; the exterior of this specimen 

 was very poorly preserved but the hinge plate was exposed, showing 

 Mulinia-like character; it seems very likely that it belongs to the same 

 species as that found at the other locality. 



Casts of a large mactroid shell, very close in outline to the speci- 

 mens referred to above, were found at University of California 

 locality 78, on the north side of Mount Diablo in the basal beds of 

 the Pittsburg tuff; however, one cannot be certain that these speci- 

 mens belong to the same species as those from the other localities. 



Family Myacidae 

 Genus MYA Linnaeus 

 MYA (CRYPTOMYA) INCOGNITA, n. sp. 

 Plate 11, figure 8; plate 14, figure 4 

 Type specimen 11114, Coll. Invert. Palae. Univ. Calif., loc. 1131 

 Shell small, thin, subquadrate in outline ; beaks near the middle, rather con- 

 spicuous. Posterior dorsal slope nearly straight, usually not so long as anterior 

 slope, which is gently convex ; posterior end obliquely truncated, on some speci- 

 mens gaping rather strongly. Ventral edge, in the middle, gently convex to 

 nearly straight, sloping up rather strongly near the ends. Surface smooth except 

 for rather fine incremental lines. On one specimen the chondrophore of the left 

 valve was exposed ; it is small with two distinct, fairly heavy ridges posterior to 

 resilium pit. The anterior of these two ridges is the raised posterior margin of 

 resilium pit; the posterior ridge extends out somewhat beyond the platform of 

 chondrophore as on Mya arenaria. Pallial sinus apparently absent. 



Dimensions. — Type specimen: length, 10.5 mm.; height, 6.5 mm.; diameter 

 of one valve, about 2 mm. 



Occurrence. — University of California locality 1131. 



M. incognita is very similar to M. calif ornica Conrad. 132 All the 

 specimens at hand of the former species are somewhat smaller than 

 the average size of the latter; outside of this, if it were not for the 

 hinge plate, it would be hard to distinguish the two species; on the 



132 For original description and figure of Mya calif ornica Conrad [originally 

 described as Sphaenia calif ornica'], see Jour. Phila. Acad. Nat. Sci., vol. 7, p. 234, 

 pi. 17, fig. 11, 1856. 



