had obtained the inner plate of the valve of Heliopelta Lewenhoeckii 

 entire and perfect. I have since found other specimens in my own 

 collection. This plate under low or medium powers shows only ex- 

 quisitely fine lines ; but with a high power (Jj) it is resolved into 

 minute sphei-ical granules of silex, arranged in parallel rows, radi- 

 ating toward the margin of the disc, placed in contact with each 

 other, and cemented together at their peripheries, the cement filling 

 the interstices. There is a distinct line corresponding to the divisions 

 of the compartments of the outer plate ; a triangular blank at the 

 junction of these lines with the margin, a conspicuous feature in the 

 view of the perfect frustule ; a star-shaped blank in the centre, the 

 rays of the star being in number one-half of that of the compart- 

 ments of the disc. Heliopelta has the disc divided into six to twelve 

 rays or compartments, one-half of them having distinctly hexagonal 

 areolae, the alternate half having an entirely different kind of mark, 

 which has never been perfectly described or figured. Dr. Carpen- 

 ter's description is, perhaps, the best, but his figure is one of the most 

 inaccurate. (Carpenter on the Microscope, Phila., p. 290.) The 

 blank star of the inner plate is also a conspicuous feature of the per- 

 fect disc, and the rays of this star always coincide with the compart- 

 ment last described. The inner plate also shows marks indicating 

 the position of the marginal (improperly so called) spines ; and under 

 a high power shows also faint impressions of the areola of the outer 

 plate, which I consider proof that the two plates were in actual con- 

 tact. It is this inner plate that gives the veiled appearance to this 

 and other Diatoms, and I take the " veil " in all cases as a visual proof 

 of the existence of the inner plate. Dr. Carpenter says of Heliopelta, 

 that a minute granular structure may be shown to exist over the 

 whole of the valve, — " that the circular areolation exists in a deeper 

 layer of the silicious lorica." 



Now, I am certain that Dr. Carpenter was mistaken in this last 

 remark, though, perhaps, not in what he saw. He had simply 

 observed a valve with the inside toward the eye. I have repeatedly 

 seen them in this position, and with the same effect. I have also 

 found what I take to be the inner plate of an Omplialopelta entire ; 

 but the evidence of its connection with that genus is not quite 

 complete. 



A few weeks since I found a broken specimen of Coscinodiscus ; 

 the hexagonal areote were large and distinct, and extending beyond 

 the broken edges, just as described in the Heliopelta, was another part 

 of the disc which was simply granular, with a milky aspect. This is 

 the inner plate of the valve of that genus. Since that I have found 

 numerous examples of the same kind, and am now satisfied that they 

 are quite common, and that others as well as myself must have seen 



