26 PANDORID,E. 



ser. Zool. ii.), remarks with respect to this species, that 

 the outer gill, which resembles a hood, might at first 

 sight be taken for a single leaf, so disproportionately 

 small is its size. He considers it a case of arrested 

 development. Mr. Jordan says, "Whilst collecting 

 specimens at Jersey, I noticed that they have a habit of 

 squirting, like Saxicava rugosa and the Pholades when 

 first touched; one individual ejected a fine stream, fully 

 sixteen inches high." In Mr. Clark's description of the 

 animal of var. obtusa the tubes are stated to be fringed 

 at their orifices with fine white short cirri ; the margin 

 of the sheath, in some specimens, is marked by a fine 

 orange line; and the base of the cirri and margins of 

 the orifices are usually encircled by a dead-white narrow 

 thread. The ovary is of a reddish-brown colour. I 

 found it to contain in July an immense mass of vesicular 

 ova in different states of growth ; the more forward of 

 them resembled in shape some species of Cy there. Adult 

 specimens vary in their comparative length and propor- 

 tions, as well as in the prominence of the ridges on the 

 dorsal side. The difference between the typical shell 

 and the variety obtusa apparently arises from the nature 

 of their respective habitats the one being sublittoral, 

 and the other belonging to deeper water. An inter- 

 mediate form has been taken by Cailliaud on the coast 

 of Brittany, and by M' Andrew at Corunna. On a 

 superficial view, indeed, it would seem as if a valid 

 distinction existed in the length from the beak to the 

 front margin being always greater in P. incequivalvis (or 

 rostratd), and on the posterior side in P. obtusa ; but 

 this only shows that varieties, as well as species, have 

 some one character of their own. Such may be expected 

 when the conditions of life vary. The extension of the 

 posterior side in the typical form may be caused by the 



