348 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
mainly of aragonite, the crystals of which appear to inter- 
lace and to be arranged very irregularly (Plate I, 
fig. 2). 
In transverse sections across the ribs (fig. 2), the 
flattened crystals are laid down so that the structure 
appears to be lamellar, somewhat like the nacreous layer 
of other Lamellibranchs. These lamellae run practically 
parallel to the surface of the shell, and each rib is formed 
by a great thickening of this lamellate layer, the lamellae 
being arranged to form two crests as figured. ‘The 
structure of the shell between two ribs is more irregular, 
and recalls the geological structure known as_ false 
bedding—the laminae lying in various planes. 
While the median portion of the thickness of the 
shell is as described above, the external surface layer is 
formed of crystals which are arranged in some places 
perpendicularly, or nearly so, to the surface of the shell, 
and in this way a kind of pseudo-prismatic layer is built 
up, but it passes gradually into the coarser and more 
irregular layer below. The inner surface of the shell is 
also laminar in structure, the laminae being practically 
parallel to the surface. If the shell sections be cut 
through the adductor impression, a thin, delicate layer 
(Pl. Il, fig. 2, SA. m.) will be found situated between the 
inner lamellar layer and the adductor muscle. This is 
the limy-looking layer seen in surface view of the muscle 
impressions, which sometimes adheres to the muscle and 
can be pulled away with it. It is best seen in sections 
through a young Pecten, the shell of which has been 
decalcified. This layer appears to be made up of numerous 
fine rods placed side by side, vertical to the shell surface. 
In sections of older shells, the rods are not so distinct, 
but the layer shows very definite striae perpendicular to 
its surface. It 1s by means of this “ Durchsichtige 
