46 



old-established cases that grooves and tubular openings are found, 

 and there they only exist on the margins as a rule. The above 

 remarks apply to the old or thick parts of the valves ; the newer 

 thin deposit over the mud, as before mentioned, merely exhibits 

 the irregularities of the surface over which it was laid whilst in a 

 soft pliable condition, and is usually without any trace of grooves, 

 except near the margin. Even these grooves, when examined with 

 a lens, show a mould of what was beneath, without exhibiting 

 any signs of having been bored. Another feature is the entire 

 cavity occupied by the mud and worm, which cannot be accounted 

 for by the boring theory. If the worm bores into the substance 

 of the shell, how are the blister-like cavities formed ? It is not 

 reasojiable to suppose that the worm has the power of raising a 

 rigid layer of shelly matter and forming a blister. To do this 

 the layer must be rendered pliable, otherwise there would be 

 evidence of such raising in the shape of cracks, etc. If the 

 blisters are formed by the disintegration of the shell, there ought 

 to be some evidence on the inner surfaces ; but there is nothing 

 to show that disintegration had taken place. One surface is 

 comparatively smooth, and the other a perfect mould of the 

 enclosed mud. 



Is it not more reasonaljle to suppose that the upper layer 

 is deposited over the mud whilst in a soft state, simply covering 

 the mud and worms, than to suppose that the worm bores into 

 the shell and then forms the l)lister 1 If the blisters were formed 

 by the disintegration of the shell, there ought to be some variation 

 in the thickness of the layer, inasmuch as the disintegration would 

 be unequal, and be most evident immediately over the worm. 

 Such, however, is not the case ; the deposited layer is pretty 

 uniform in thickness over each blister. 



From an examination of a large number of shells in sections, 

 it appears to me that the cavities when once formed are never 

 enlarged in any perceptible degree. Frequently, when viewed in 

 section, cavities may be seen one above another in tiers, each one 

 distinct, but regulated in form by the one below. These various 

 cavities simply represent the entrance of so many worms into the 

 open shell at dilFereut times, each worm in turn being covered 

 over by a thin deposit. 



Mr. A. Oliver, in an article in the " Centennial Magazine " for 

 September, 1889, suggests that the death of the oyster takes place 

 from being unable to close the valves on account of the under- 

 mining of the attachment of the abductor muscle. I may say 

 that during the whole of my investigations I never met with such 

 a case, a fact which militates against the boring view. I, how- 

 ever, met with many instances in which the muscular spot was 

 considerably lessened, not by its being undermined, but by the 

 encroachment of the worm around the point of muscular attach- 



