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SIDNEY F. HARMER. 
so close together that the interposed calcareous substance 
finishes by disappearing, and the plant thus conies into 
contact with the external water, and is able to discharge its 
reproductive cells. The surface of the shell is thus rendered 
rugose and uneven. This process is supposed to be the 
principal cause of the disappearance of empty shells in quiet 
bays. 
I have not succeeded in determining the vegetable organisms 
found with the Ph or on is oval is, though they appear to have 
some resemblance to the Alga described by Bornet and 
Flahault as G-omontia polyrhiza. The hyphae of this 
plant are said to have a maximum diameter of 12 /u — a size 
which is considerably exceeded in the largest filaments found 
in the Northumberland material. 
The thickness of the N eptunea-shell varies between about 
2 and 4’5 mm. In the neighbourhood of the columella it 
reaches its greatest thickness, while it is much thinner in the 
middle of the whorls. The diameter of the tubes of the 
majority of the Phoronis individuals is from *250 to 
*275 mm. Even in the thinnest part of the shell the 
diameter of the burrow of the Phoronis is thus not more 
than about one-eighth of the thickness of the shell, and there 
is accordingly plenty of room in the substance of the shell 
to accommodate a large number of these burrows. 
The general arrangement of the cavities inhabited by the 
Phoronis maybe indicated by comparing the shell with a 
mass of wood excavated by the burrows of Teredo. The 
Phoronis is present in very large numbers, its burrows 
passing in all directions through the shell, and opening to the 
exterior either on the outer side or on the inner side. 
The distal end of the burrow is commonly placed at right 
angles to the surface, but in some cases part of the tube lies 
in a superficial groove of the shell. In addition to the 
Phoronis and the Alga already mentioned, the substance 
of the shell is inhabited by other boring organisms, and 
particularly by the. Sponge C lion a and a Polychaete which 
