442 Royal Society. 



parabola and a flat Lieberkuhn, as the writer has elsewhere de- 

 scribed*. The underside of the scale thus appears as a smooth 

 glistening surface with very slight markings, corresponding probably 

 to the points of insertion of the plates on the contrary side. The 

 minuteness and close proximity of the epithelial plates will readily 

 account for their being a good test of definition, while their promi- 

 nence renders them independent of the separating power due to large 

 angle of aperture. 



The structure of the second class of test- objects above mentioned 

 differs entirely from that above described ; it will suffice for the pre- 

 sent purpose to notice the valves of three species only of the genus 

 Pleurosigma, which, as arranged in the order of easy visibility, are, 

 P.formosum, P. hippocampus, P. angulation. 



These appear to consist of a lamina of homogeneous transparent 

 silex, studded with rounded knobs or protuberances, which, in 

 P.formosum and P. angulatum, are arranged like a tier of round shot 

 in a triangular pile, and in hippocampus, like a similar tier in a qua- 

 drangular pile, as has frequently been described ; and the visibility of 

 these projections is probably proportional to their convexity. The 

 " dots " have by some been supposed to be depressions ; this how- 

 ever is clearly not the case, as fracture is invariably observed to take 

 place between the rows of dots, and not through them, as would na- 

 turally occur if the dots were depressions, and consequently the sub- 

 stance thinner there than elsewhere. 



This in fact is always observed to take place in the siliceous loricse 

 of some of the border tribes that occupy a sort of neutral, and not 

 yet undisputed, ground between the confines of the animal and vege- 

 table kingdoms ; as for example the Isthmia, which possesses a reti- 

 culated structure, with depressions between the meshes, somewhat 

 analogous to that which would result from pasting together bobbin- 

 net and tissue paper. 



The valves of P. angulatum and other similar objects have been 

 by some writers f supposed to be made up of two substances pos- 

 sessing different degrees of refractive power ; but this hypothesis is 

 purely gratuitous, since the observed phenomena will naturally re- 

 sult from a series of rounded or lenticular protuberances of one 

 homogeneous substance. Moreover, if the centres of the markings 

 were centres of greatest density, if in fact the structure were at all 

 analogous to that of the crystalline lens, it is difficult to conceive 

 why the oblique rays only should be visibly affected. When P. hip- 

 pocampus or P.formosum is illuminated by a Gillett's condenser, with 

 a central stop placed under the lenses, and viewed by a quarter-inch 

 object-glass of 70° aperture, both being accurately adjusted, we may 

 observe in succession, as the object-glass approaches the object, first 

 a series of well-defined bright dots ; secondly, a series of dark dots 

 replacing these ; and thirdly, the latter are again replaced by bright 

 dots, not however as well defined as the first series. A similar suc- 



* See British Association Reports for 1850. 



t Vide Quarterly Journal of Microscopical Science, No. V. pp. 9, 10. 



