John Young — Shell Structure of Bicluoaldia. 217 



through mineralization of the shell. Such a weak line I have found 

 in the shells of Productus, existing between the perforate and im- 

 perforate layers. 



In a vertical section through the thickness of the shell, the outer 

 celluliferous and inner dense layers are seen to be very distinct ; but 

 thei'e is no apparent line of separation of the shell substance between 

 the two layers. I have examined carefully under the microscope the 

 minute circular tubes that are seen in the inner dense layer, but I have 

 failed in finding evidence that they extend upwards in the shell 

 structure beyond this layer. It is probable they do not, as they do 

 not show themselves in the cross or tangential sections I have made, 

 and in the one vertical section of the shell that I have prepared, they 

 do not appear to exactly correspond, either in number or position, 

 with the larger cells that lie over them. This, however, is a point 

 that can only be satisfactorily determined by the examination of, 

 more numerous sections of the shell. 



There is less difficulty in explaining the difference of structure 

 that exists between the outer and inner cell layers. In these, we see 

 that certain of the small polygonal cells, in regular spots, ceased to 

 be further developed, while other of these cells continued to grow a 

 little higher along certain lines. These cells of the second layer were 

 afterwards covered and concealed by the growth outwards of a 

 boundary wall, that in the latest stage separates the hexagonal cells 

 from one another. This is well illustrated in a partly eroded shell 

 from Dudley in Dr. Davidson's collection, and may be shown to 

 exist in every specimen in which the shell structure is not too much 

 crystallized, if a portion of the surface be ground down a little way 

 so as to show the inner cell layer. 



Xo°Q 



12 



1. Hexagonal cells of the outer surface of the shell in unworn specimens. 



2. Small polygonal cells in walls of hexagonal cells as seen in worn or polished 



specimens. 



3. Polygonal cell layer between outer hexagonal cells and inner dense layer. 



4. Perforations of inner dense layer of the shell. 



5. Vertical section of the shell ; a, outer hexagonal cell-walls ; b, polygonal cell- 



layer ; c, inner dense layer with minute perforations. 

 All the sketches more or less magnified. 



There is no evidence of perforations in this outer shell wall, and I 

 have only to remark, before bringing these notes to a close, that a 

 farther examination of the outer cell-layers around the bare spot on 

 beak of the ventral valve, confirms what has been already stated, as 

 to the probability of that portion of the shell having once been 

 attached, and afterwards broken away from its attachment ; for 

 besides the rough, ragged, edges of this cell-layer, that I have 



