148 DR. COBBOLD ON THE EYE OF THE COD-FISH. 
attention to the first only of these complicated layers, I have to re- 
mark that in the Cod, as also in its allies, we find Jacob’s membrane 
to consist of rods and cones, the latter presenting the characteristic 
twin or double form well known to occur in many fishes. In my 
opinion this twin character is a normal condition ; but this view is 
opposed by Mr. Nunneley, who has arrived at very different con- 
clusions not only in this particular, but also in regard to their size 
and other easily ascertainable facts. According to my own ex- 
aminations, the twin-cones of the Cod present an average length 
of ;45th of an inch in length, and ;4,th of an inch in breadth; 
but I have found some only ¢1;th of an inch long, and others as 
much as z},th after they have imbibed fluids added to them. 
In the fresh condition they display the form shown in the accom- 
panying woodcut (a), having, as Hannover has remarked, the shape 

Twin-cones of the retina of the Cod, showing the formation of bacillar appen- 
dages, and the changes of form which the cones undergo on the addition of 
water. 
of a coffee-berry, the upper or inner pole of the twin-cone being 
somewhat more broadly truncated than the lower or outer pole, 
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