mainly of aragonite, the crystals of whicli apr)ear to inter- 

 lace and to be arranged very irregularly (Plate II, 

 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 tw'o ribs is more irregular, 

 and recalls the geological structure know^n 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 II, fig. 2, Sh. 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 is by means of this " Durchsichtige 



