PULLASTRA. 119 
open, having the anteal and posteal portions of the margin 
dentated, the middle is smooth and gently sinuated. The 
siphons, when extended, are fully as long as the shell, and 
united to about + of an inch from their extremities, when 
they become well separated, the anal one curving upwards, 
the branchial inclinmg downwards; the former is circled with 
20-30 pale brown short cilia, sometimes tipped with white, 
the latter with a variable number of longer brown or pale red 
ones, each being fringed on both sides with short horizontal 
white fimbriz; there are also 16-30 shorter imtermediates, 
some plain white, some brown and fimbriated on both margins; 
the siphons are white, except a short area at their termi- 
nations below the cilia, of red-brown dark bistre, or of those 
colours mixed, and marked with short fine red transverse 
Imes and longitudinal blotched ones. A pair of pale brown 
branchize on each side; the upper laminz not covering the 
lower, decussated on both surfaces by the network of the 
blood-vessels ; the palpi are small, triangular, striated on the 
inner surface, and smooth without ; the posterior ends of the 
branchize are permanently fixed in this species and the P. de- 
cussata to the roof of the branchial tube, as in Pholas. 
The Venus perforans of authors is only a dirty-white or 
ochraceous specimen of this species, arising from the exclusion 
of light, in consequence of its habitat being adventitiously cast 
on rocks, where, like the Savicave and other excavators, it 
has the power of imbedding itself. We have carefully ex- 
amined and compared with them many of the free shells from 
the shingles, and have not detected the slightest specific 
difference: we consider the two identical. The foot of this 
species is pure milk-white, with a byssal groove producing 
fine filaments, which, when the animal is not in pure sand, 
attach it to the shingle; it is muscular, slightly geniculated, 
and lanceolate. 
This is strictly a littoral species, never being met with in 
the dredge, and is scarce on the shingle tracts at Exmouth. 
The variety, the V. perforans of authors, is abundant, being 
imbedded with the Pholas dactylus in the red sandstone at 
the same place. 
