CLASS PELECYPODA 
217 
one-third as long as the ventral. The anterior median part is marked by 
rather broad, strongly denticulated ridges which join those of the anterior 
part in a little more than a right angle. The junction of the anterior dental 
ridges and those of the posterior-median part form a well-impressed line 
which extends from the ventral margin to the umbos. The spaces separat¬ 
ing the dental ridges on the anterior median portion are mere impressed 
lines. The anterior half of the median portion is convex, while the 
posterior half is depressed, forming a groove. Both parts are crossed by 
rough, cord-like incremental lines, which follow a more or less sinuous 
course. The posterior portion of the median part is about as wide as the 
anterior and the median portion taken together, and is crossed by oblique, 
somewhat wrinkled lines of growth. The posterior part forms a strong 
auricle, which is thin, decidedly expanded and slightly reflected at the 
margin, and emarginate at the umbonal terminus. The interior is bluish 
white. The umbo marks a strong knob from the underside of which the 
broad blade projects for a little more than half the length of the shell. 
The ventral portion of the median part bears the usual strong knob. The 
anterior and the median part show a strong tumid area at their junction, 
while the median portion of the middle part is concave. The posterior part 
projects over the posterior median parts as a strong shelf. This part in 
cross section would show a decidedly sinuous outline. The emargination 
of the dorsal portion of the auricle shows best when seen from within. 
The pallets form long, plume-like elements composed of a series of 
cup-in-cup structures, which project at the lateral margin in the form of 
strong awns. Each cup is covered by a thin periostracum which is faintly 
fimbriated at the distal end, and marked by longitudinal striations. The 
stalk is about one-fourth the length of the entire pallet. Height, 11; 
length, 12.5; diameter, 11 mm. The pallet measures, entire length, 32.5; 
length of stalk, 7.5; diameter, 3.7 mm. (Bartsch.) 
Type in U. S. N. M., No. 102762. Type locality, San Francisco Bay, 
California. 
Range. Unalaska to San Francisco, California. 
Genus TEREDO Linnaeus, 1753. 
In this genus the pallets are either paddle- or spoon-shaped. They 
may be distally cupped or not, or they may even bear a calcareous knob 
at the terminal portion. (Bartsch.) 
Type. Teredo navdlis Linnaeus. 
Distribution. Norway, Britain, Black Sea, Tropics, Atlantic and 
Pacific Coast of America. 
Range in time. From the Lias. United States, Europe. 
