402 Transactions of the Society. 



it has liitbcrto l>eon disputefl, further delate as to its existence can 

 hardly be regarded as tenable. Grunow has figured it in several 

 varieties of G. aster omphalus Grun., and in C. excentricus and 

 C. lineatns Ehr.* Besides verifying it in these, I have observed and 

 noted it in C. ocuhis iricUs Ehr., C. radiatus Ehr., C. perforatus 

 Ehr., C. centralis Ehr., Sifstej^hnnia diadema and S. corona, 

 Ehr., in Endyctia oceanica Ehr., and Eupodiscus radiatus Bailey. 

 In some of these cases it is only the marginal circlet of dots within 

 the hexagon which can be clearly seen, but after the preceding 

 steps in the proof, these will be regarded as sufhciently conclusive 

 marks of the presence of a covering film. This typo of construction 

 is therefore probably common to all the disk forms of the Diatom- 

 aceae having large hexagonal alveoli. It may be well to emphasize 

 the assertion that in these disk forms the " eye-spot " film is invari- 

 ably that which is upon the inner or concave surface of the valve, 

 and it is the outer which, when thickened, shows the system of dots 

 within the hexagons and to which the hexagonal walls of the alveoli 

 are permanently adherent.f In Triceratinm favus, on the other 

 hand, it is the inner film which is the heavier and contains the 

 radiating system of dots. It is to this that its hexagonal walls 

 are permanently attached. 



It is no uncommon thing to find broken specimens of T. favus 

 which give us natural sections as perfectly made as any that could 

 be produced by cutting. The strong marginal band of the valve is 

 very often fimbriated, and the fimbricne show a tendency to become 

 confluent. This gives an increased breadth to the band, and it now 

 projects almost equally above and below the floor of the valve 

 containing the hexagons. The shell often breaks upon a line 

 parallel to the marginal band and through the row of alveoli nearest 

 to it, giving a perfect section through these hexagons. In a 

 mounted preparation the fragment will adhere to the cover-glass by 

 the broad band which is so transparent that one may easily focus 

 through it upon the walls of the hexagons thus presented in section. 

 In such cases these walls appear as a line thickened into a knob at 

 the top, whilst the dotted iloor of the alveoli is shown as a con- 

 tinuous Une at right angles to the walls.J In most of such 

 examples the thinner outer film cannot be traced. In others it 

 seems to be present ; but as in the latter examples there may be 

 doubts wliether the section does not include more than one row of 

 alveoli, the evidence from this source cannot be regarded as conclu- 



* Van Heurck's « Synopsis dcs Diatomces de Belgique,' 1884, pis. 128, 130, LSI. 



t In his drawings of sections of diatoms Dr. Fltiirel would seem to make the 

 "eye-spot" film of Coscinodiscus rculi'itiis tiie outer film of the valve. See this 

 Journal, iv. (1884) pi. ix., figs. 23,24,20. I have never found it in that situation. 



X Photographs Nos. 100, lOG. 



