126 



Prof. W. King on the Histology of 



Fig. 3. 



render it a matter of surprise how its perforated character can 

 be doubted. 



From the surface of the large valve, a little in front, and on 

 the left of the part where the umbone is broken, some shreds 

 have been removed, apparently for microscopic examination. 

 This portion of the specimen appears to be so much altered by 

 mineralization as to render it, in my opinion, of little or no ser- 

 vice in settling the question at issue. On those parts from which 

 the shreds have been removed the black dots are distinctly 

 visible ; and the remaining test on which they are seen is of 

 such a thickness as to completely preclude the idea of their being 

 " only pits upon the internal surface of the shell." 



2. The nearly perfect specimen. — This specimen is exceedingly 

 valuable, inasmuch as it decisively exhibits, on the left side and on 

 the umbone of the large valve, the original or outer surface of the 

 shell. All the other parts are 

 either obscured with foreign 

 matter, or they show that por- 

 tions of the shell-substance 

 have been accidentally worn 

 down or otherwise removed. ^' 

 The black dots are distinctly 

 seen where the abrasion oc- 

 curs; and in two or three 

 places on the umbonal region 

 of the small valve they are 

 visible on at least three dif- 

 ferent layers, a, b, c, as repre- 

 sented in fig. 3. The parts 

 marked with an asterisk con- 

 sist of a mere crust of mineral 

 matter, which appears to have 

 replaced the original surface- 

 layer. A portion of the right 

 half of the anterior region of 

 the shell, where the margins of 

 the valves join, also exhibits 

 black dots very distinctly, as represented in fig. 4 : the test is 

 evidently of considerable thickness at this part. 



With respect to the umbone of the large valve, it does not, to 

 me, show any black dots, either on the original surface or in 

 the tissue beneath it. Whether this absence is due to the shell- 

 substance of the umbone having undergone a molecular change 

 or metamorphisra, or has been produced by any other cause, 

 are points on which I cannot offer any decided opinion*. I am 

 * To show the effects of fossilization, I may mention that the type 



