216 John Toimg — 5'/?^// Structure of Eichwcddia. 



to slight change through the action of mineralization, as noticed in 

 some other fossil shells. 



My examination of this beautiful and interesting shell has led me 

 to recognize three distinct layers in its structure as shown in the 

 annexed woodcut : — First, the outer hexagonal cell layer, in which 

 the walls are thin at the surface, but thicker at their base, and cover 

 a layer of minute polygonal cells, that are bounded by thin walls. 

 These polygonal cells form the second layer inwards, and are not 

 seen on the surface, except where the outer cell layer has been eroded 

 or not well developed. A certain number of these cells rise above 

 the bottom of the larger cells, and enter the outer cell-walls. In 

 eroded or polished specimens, as many as from six to eight of these 

 smaller cells may be seen in the wall around each larger hexagonal 

 cell, one being generally situated in each of the angles, and some- 

 times one between ; but, as already stated, they do not appear to reach 

 the outer surface of the shell in perfect or unworn specimens. 



The third or innermost layer of the shell is the dense portion that 

 is perfoi'ated by the minute circular tubes. These tubes are as small 

 as those seen in the shell structure of the Carboniferous Terehratula 

 liastata, but they are only about one-third as numerous as seen in 

 that shell. They agree in character with that seen in many other of 

 the Brachiopoda ; but the outer shell layers of E. Capewelli, Dav., are 

 so unlike what is known to exist in this group, that one is not sur- 

 prised to learn that they have been suspected to be of Polyzoal 

 origin, and the resemblance of the cell arrangement to that seen in 

 some forms of Monticulipora is further increased by finding that 

 the outer cell-walls enclose a series of smaller polygonal cells, as is 

 common in some of the members of this group. I do not, however, 

 know any form of Silurian Polyzoa or Monticulipora, that has exactly 

 the same extei'nal form of cells, and ray examination of E. Capeioelli, 

 strongly inclines me to the belief that, although the structure is 

 peculiar for a Brachiopod shell, yet it is one that naturally belongs 

 to it, although, at first, I was inclined to regard it, as some others 

 have done, as probably Polyzoal. 



As just stated, 1 am inclined to regard the external celluliferous 

 structure of E. Capewelli as natural, and have been gradually led to 

 this conclusion by the careful study of Dr. Davidson's specimens, 

 and from sections of portions of the shell that I have made. 



If the outer celluliferous . layer is examined, it will be observed 

 that it is thin at the beak of the ventral valve, where the bare 

 triangular spot is seen. From this point it very gradually increases 

 in thickness as it reaches the front margin of the shell. In a 

 vertical section, through both valves, and from fragments picked 

 from the shale, I find that the outer celluliferous layer is just about 

 equal in thickness to that of the dense inner layer of the shell. 

 From the way in which this outer layer has parted from the inner, 

 in every instance, on the bare spot on the beak of the ventral valve, 

 it would almost lead to the supposition that, previous to fossilization 

 of the shell, along this line there was a weak connectioa 

 between the two layers, although now they seem to be firmly united; 



