structure of the Scales of Lepisma Saccharina. 15 



like structure on or near the other surface. Careful focussing is 

 corroborative of this idea, making it certain that these two details 

 of structure lie in different planes. With monochromatic light, 

 the delineation of this structure is eminently satisfactory, and the 

 effect of the slightest change in focal adjustment is at once felt. 

 When the object is a little out of focus the light is unequally re- 

 fracted and broken up in passing through this complicated net- 

 work of ridges and corrugations, and produces an appearance of 

 fine molecules over the whole surface of the scale. 



The coarse and the fine beads both vanishing under advancing 

 definition, together with the behaviour of the confined bubbles of 

 air, seems to my mind fully to demonstrate the reality of the 

 structure above described. Often, when the corrections are not 

 perfect, the semblance of beading can be directly traced to a seem- 

 ing enlargement of points of linear intersection and branching. 

 When the sVth is at its best work the finer transverse markings are 

 usually irregular both in strength and direction, but always unmis- 

 takable. They may be plainly seen on some of the smaller scales 

 and in the central parts of the larger, and at almost as good 

 advantage as near the edges of the easier scales. Sometimes they 

 are continuous across several intercostal spaces, and again only 

 extending across one, or it may be merely budding, as it were, 

 from the ribs. It will be noticed that the " beads " as drawn by 

 Mr. Hollich exhibit corresponding irregularities. 



In conclusion, the remark of Beck on the scales of Lepido- 

 cyrtus may well be quoted — " and my own belief is that the mark- 

 ings upon this and all other varieties of Podura-scales are more or 

 less elevations or corrugations upon the surface, which answers the 

 simple purpose of giving strength to very delicate membranes."* 

 If this idea is true of the Podura, it applies with greater force to 

 the complicated ridges of Lepisma. 



The same original structure is often modified in diverging 

 directions so as to subserve totally distinct purposes. And as hairs 

 are probably modified scales, and a regular gradation may be traced 

 between them, so the connecting chain is filled up between ribs 

 extending from end to end of a scale, through undulations and 

 shorter ribs, to those slightly projecting, and so on to the perfect 

 spine or secondary hair. — The American Naturalist, Nov. 1873. 



* ' Transactions of R. M. S.,' 1SG2, p. 83. 



VOL. XI. 



