1918] Packard: Molluscan Fauna from San Francisco Bay 223 
from the Atlantie coast. One of these, Arca transversa Say, has not 
heretofore been reported from the West. It is represented in the 
Survey collections by a number of old valves. Living specimens have 
not as yet been taken, so it is not certain that this common Atlantic 
form is now living within San Francisco Bay. Australium triumphans 
Philippi was dredged alive just outside the Golden Gate. Although 
the locality lies within the course of oceanic traffic, the probabilities 
are against the dredging of a single specimen that may have been 
carried across the Pacifie and then dropped from the bottom of the 
ship. If this species is exotic, it presumably has established itself 
within these waters. The eastern oyster, Ostrea elongata Solander, 
more generally known as O. virginica, is grown within San Francisco 
Bay. It does not reproduce, at least sufficiently for economic pur- 
poses, so young oysters are introduced from the east to replenish the 
beds, depleted by the annual harvest. Mya arenaria Linnaeus, the 
“soft shelled’’ or ‘‘mud eclam,’’ is thought to be exotic. It is not 
known to be cireumpolar in distribution and does not occur native in 
Puget Sound. That it is not indigenous to San Francisco Bay ap- 
pears to be indicated by the lack of shells of this edible clam in the 
undisturbed Indian shell mounds that oceur in the vicinity of the bay. 
Urosalpine cinereus Stimpson was first noted by Stearns (1894, p. 94). 
It is now abundant in the lower division of the bay in the vicinity of 
Point San Mateo, where it plays havoe with the oysters. Another 
predatory gastropod, Zlyanassa obsoleta Say, first appeared in 1909, 
according to Keep (1911). As yet it is confined to the southern part 
of the bay, where it is associated with the preceding species. Modiolus 
demissus (Dillwyn), another eastern form, was first reported by 
Stearns in 1899. It has now established itself in these waters, and it 
is occasionally found in the San Francisco markets. The venerid, 
Gemma gemma var. purpura Lea, known here since 1899, now occurs 
in great numbers within the shallow waters along the southeastern 
shores of the bay. 
The tubular bottom samples have revealed the fact that conditions 
have not been equally favorable to mollusean life during different 
periods of time. Sumner et al. (1914, pl. 6) have shown that in cer- 
tain regions of the bay, notably south of Hunters Point, in the lower 
division, the stratified samples contain layers of shell, covered by a 
superficial deposit of nearly pure mud. Of the ten sections shown in 
diagram for the region south of that point only one (H 5300) is rep- 
resented as having an appreciable amount of calcium carbonate (shell) 
