1918] Packard: Molluscan Fauna from San Francisco Bay 269 
Saxidomus Conrad 
Saxidomus nuttalli Conrad 
Plate 21, figure 2 
Saxidomus nuttalli Conrad (1837), p. 249, pl. 19, fig. 12; Dall (1902), p. 391. 
Saxidomus aratus Gould, Carpenter (1863), p. 641; Arnold, R. (1903), 
p- 151. 
Saxidomus giganteus Deshayes, Dall (1902), p. 391. 
Saxidomus squalidus Deshayes, Carpenter (1863), p. 641. 
Description.—This species was described by Conrad (1837) as follows: 
‘*Shell suboval; disk rough, with concentric striae, elevated on the posterior 
slope; posterior extremity truncated; colour white, with brown spots and stripes 
about the umbo and ligament margin.’’ 
Length, 15 to 105 mm. 
Occurrence.—At stations D 5701 (1), D5702? (6), D5703* (2), 
D 5779 (1), D5801 (15), D 5827 (1), D 5827 A (1), D 5829 A (2), 
D 5842 (1), D 5843 (8), and questionably at D 5704, D 5744, D 5809, 
D 5825, D 5844. 
This species has heretofore been considered as occurring mainly 
south of San Francisco. It has been thought to differ from a northern 
form known as Saridomus giganteus in its more pronounced concentric 
ribbing and in the purple coloration of the interior of the shell. -The 
identification of the specimens obtained by the Survey led to an 
examination of a large number of fossil and Recent specimens from 
various localities. Measurements and a detailed study of the promi- 
nent shell characters of the Recent specimens showed that there was 
such a commingling of characters as to indicate that the northern and 
southern forms were not separable. The purple coloration, thought to 
be characteristic of the southern form not only has been reported on 
specimens from Puget Sound but has since been found by the writer 
on a typical specimen of the northern form obtaimed from Sylvan, 
Washington. Dr. B. L. Clark and the writer in examining the Mio- 
cene forms found gradational types connecting forms as divergent 
as any occurring at the present time. Clark recognizes, therefore, but 
the one species in his recent paper (1915, p. 420). 
The single living specimen was dredged in 814 fathoms in the mid- 
dle division of the bay. This species occurs upon gravelly bottoms, 
being among the list of those prevalent for that bottom group. It 
is confined in its local distribution to the middle division of the bay, 
occurring more abundantly in the outer portion of the Golden Gate. 
Range.—Aleutian Islands, Alaska to San Diego, California. 
