682 JAMES L. KELLOGG cern  £ fake 
toward the lateral oral groove; and no dorsally or ventrally 
directed currents were found between folds. Only two speci- 
imens were available for examination, however, and it may be 
that such tracts exist. 
Zirfaea gabbi Tryon 
The shell of one specimen of this Pholad taken from Puget 
Sound measured 12 cm. or nearly five inches in length. The 
animals were dug with a pick from blue clay, nearly as hard as 
rock, and the burrows of larger individuals were about 20 inches 
in depth. Siphon and gill development here, and in some other 
members of the family, are very extraordinary, as shown in figure 
54. The mantle chamber cannot be distinguished from the 
incurrent siphon tube, the whole mantle being extended poste- 
riorly without the usual constriction of the siphon base. Instead 
of ending posteriorly at the base of siphon tubes, as is usual, the 
gills extend backward, in an uncontracted siphon, for nearly 
two-thirds of its extent, attaining an actual length, in large indi- 
viduals, of a foot or more. The tubes of the distal third of the 
siphon are separated by a muscular septum (ms), the dividing 
wall of the remainder being formed by the gills which are united 
on the median line and joined laterally to the siphon wall, as 
shown in figure 56. In this figure the excurrent siphon is cut 
and opened dorsally. Possibly the distal region only should be 
regarded as true siphon, but a strong argument could be formu- 
lated against such a view. Mantle and siphon walls are of great 
thickness. There is a piston-like foot which may in some way 
aid in boring; but it may be that the great muscular develop- 
ment of the siphon enables the animal to impart a drilling motion 
to the anterior part of the body when the structure fits tightly 
in the burrow. It would be interesting to know how, during its 
growth, this and other borers manage to deepen and enlarge 
their burrows. 
Gills. On all lamellae of these enormously extensive col- 
‘ectors, currents trend ventrally to the margins, where material 
is received into grooves. Apparently the conduct of material 
