THE BRITISH FOSSIL BRACHIOPODA. 281 



be very difficult to say, as the openings appear to have been too small to admit any 

 foreign mineral matter into the substance of the shell, by which such external openings 

 could be readily recognised from the ordinary calcite of the shell. 



" When a specimen is examined with a pocket-lens, and is held so that the light shall 

 pass through the ribs cross-wise in the direction of their length, the perforations are then 

 seen to extend downwards into the substance of the shell through a layer representing 

 about the thickness of the ribs [fig. 21 a\ When looked at in a vertical direction under 

 a low power of the microscope, they appear on the surface of the ribs as minute rows of 

 tubercles, slightly raised above the level of the shell. These tubercles are so close to one 

 another that their bases almost touch each other, and in the layer of the shell in which 

 they have been formed it is evident that the shell-structure, during its growth, has been 

 modified to a certain extent, so as to allow of its arrangement around the perforations 

 into the form of minute tubercles. 



" This beautiful and interesting structure I have only found after etching some of my 

 finest-preserved specimens. It seems to be destroyed in those specimens where the shell 

 is much crystallised. 



" In the larger, slanting, tubular openings found on the ribs [fig. 20 b\ these, in etched 

 specimens, are seen to be filled with mineral matter, different from the calcite of the 

 shell, and stand out on the surface as small, short spines. They do not pass through the 

 thickness of the shell, but disappear, as the surface of either valve is etched, near the 

 middle layer. At this point a new series of perforations are seen in the shell-structure 

 which do not reach the outer surface but pass inwards in an upward, slanting direction, 

 and open on the interior of the surface of the valves as numerous, raised, blunt tubercles, 

 that give the inner surface quite a roughened appearance. 



" This inner series of perforations are large and wide-set, being most numerous around 

 the outer margin of the valves, and are placed in single rows between each of the ribs, 

 whereas the tubular openings or bases of the spines seen on the outer surface of the shell 

 are all planted on the ribs [fig. 21]. There can, I think, be little doubt about these 

 inner perforations being tubular, as in some specimens they are found to be filled with a 

 darker mineral matter than the substance of the shell, and in thin sections the shell- 

 structure is seen to be arranged around the openings in minute waving or concentric 

 lines of growth. This inner layer of shell around the perforations is dense in its 

 structure, and is penetrated with no smaller series of pores than those seen between the 

 ribs. In this it agrees in its inner shell-structure with several of the species of Productus, 

 in which also, as I have already shown, the inner series of perforations never did reach 

 the outer shell-surface. 



" The row of tubular spines along the cardinal edge of the ventral valve of this species 

 of Chonetes reveals some interesting evidence as to their arrangement, which I do not 

 think has been before recorded. These spines pass right through the thickness of the 

 shell, as tubes, and open on the inner surface, the tubes being often filled with foreign 



