PHOLAS. 175 



Pholades rarely exceed | 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 being 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. DACTYLUS, Linnseus. 

 P. dactylus, Brit. Moll. i. p. 108, pi. 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, enclosing 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 fimbrise, 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 papillae . 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 



