John Young — Shell Structure of EicJucaldia. 215 



and satisfactory manner ; also, that those he allowed me to etch and 

 polish showed in every instance the existence of this inner laj'er of 

 smaller polygonal cells beneath the layer of larger hexagonal cells. 



On E. Capeioelli there is a curiously bare triangular spot on the 

 back of the beak of the ventral valve, on which the external shell- 

 ornamentation is now wanting. Dr. Davidson says the smooth 

 triangular space on the beak has been noticed by several observers 

 of this shell, and that he has seen it on all the English as well as 

 foreign specimens that he has examined. Prof. Hall, as quoted by 

 Dr. Davidson, M. Brit. Sil. Brach. Sup. p. 41, says : " The small 

 triangular space near the ventral beak, which is destitute of marking, 

 has the appearance of having been exfoliated." I am inclined to 

 think that Prof. Hall is right, for an examination of this spot under 

 the microscope shows that the edge of the ornamented shell-layer is 

 torn and ragged around the triangular space in most specimens, and 

 presents the appearance as if this portion of the shell had been 

 broken awa}^ from some sort of attachment to which it adhered 

 during life. In perfect specimens of the shell, it is only on this 

 bare spot that one gets a glimpse of the inner layer of the shell, as 

 on the rest of the surface it is hidden by the outer cellular structure. 

 This inner layer is seen to be smooth, and dense in its structure, 

 and presents a striking contrast to the outer cellular layers, which 

 are of a more open texture. From the smallness of space exposed 

 within the triangular area on the beak, I was not able to determine 

 whether this inner shell-layer was perforated in any way. It there- 

 fore occurred to me that I might be able to throw some light on this 

 point, if I could examine fragments of the inner surface of the shell ; 

 and fortunately I was able to do this, by having beside me some of 

 the washed Wenlock shales in which the shell is found, kindly sent 

 to me by Mr. G. Maw, F.G.S., at the time he was investigating 

 these shales. 



From the fragments of the shell that I have obtained, I have 

 prepared two slides of specimens, which I have presented to Dr. 

 Davidson, so that he might better understand the structure seen in 

 his other specimens. One of these slides shows the inner surface of 

 the shell in the condition in which the fragments were found. The 

 other slide shows tangential sections, polished to a thin transparency, 

 so as to enable the structure to be examined under the microscope. 



So far as Dr. Davidson has been able to describe this shell, per« 

 forations of the inner shell-layer have not been noted ; but the 

 specimens I have found show that this inner layer is distinctly 

 perforated by numerous minute circular tubes, that open on the 

 inner surface of the shell, in some specimens, with slightly raised 

 tubercle-like orifices. These perforations are visible to the eye in 

 good light with a pocket lens, but are better seen under a low-power 

 of the microscope, and in the transparent sections many of the tubes 

 are seen to be filled with calcite, others with pyrites, and when this 

 is the case, they appear in the section as a series of round black 

 dots. The shell structure between the perforations is seen to be 

 dense, and has a minutely granular texture which may be partly due 



