PHOLAS. 175 
Pholades rarely exceed 4 an inch in length, consequently these 
dwarf forms are the result of locality, depth of water, and many 
other conditions. In the deeper zones, the young forms of 
the present species, instead of bemg found in the proportion 
of one to twenty of the adult shells, appear in equal numbers : 
this discrepancy in the proportions of the young shells in- 
habiting the littoral and pelagic zones, must arise from the 
circumstance that in the deeper waters there is more room for 
reproduction, more sustentation, and fewer enemies ; this view 
corroborates the doctrine above, accounting for the disparity 
of numbers in the littoral zones between the young and old 
shells of this species. 
P. pactyius, Linnzus. 
P. dactylus, Brit. Moll. 1. p. 108, pl. 3. 
Animal conically elongated ; body pale bluish-white ; mantle 
tinged with yellow, very thick ventrally, posteally, and around 
the gape; the other portions are of the thinnest texture, 
closed throughout, except anteriorly a large oval aperture for 
the passage of the foot, and having the posterior end produced 
into a long retractile sheath of a milk-white colour when 
denuded of the epidermis, enclosmg the branchial and anal 
siphons, which are just separated at their termini, the former 
with 12-16 long cirrhi, usually furnished on one side, some- 
times on both, with 3-7 fimbriz, besides one or two inter- 
mediate shorter cirrhi, which are ciliated on both sides. The 
anal tube is plain and slightly escalloped, but in some states 
it appears to have a number of short blunt cirrhi, which are 
not real, but occasioned by the doubling of the points of the 
scallops on contraction; their colour is brown, interspersed 
with a few white blotches, producing a pepper-and-salt ap- 
pearance; the sheath. for a short distance from the terminus 
is studded with subcircular whitish squamous papille. The 
foot is plain, hyaline, bluish-white, suboval, pointed before 
and behind, truncate basally, rather obliquely fixed to the 
body by a long cylindrical, thick, fleshy white pedicle. The 
sheath can be extended to double the length of the shell, and 
the branchial portion is often distended with water to three 
