18S1.] Mr Glazebrooh, On some equations &c. 155 



identical individuals lent to Sowerby for figuring, it is quite plain 

 that they would be the same species. Hence C. Vectensis, Morris, 

 must rank as a synonym of G. nitida, Sow., which was collected at 

 Hamstead. 



Deshayes in 1824 gave the name G. nitida to a shell from the 

 Calcaire grossier [Goq. foss. Env. Paris, I. p. 57, pi. 8, figs. 39 — 41], 

 but as he removed it subsequently to the genus Sphenia there is 

 no question of ambiguity. 



Having to give a new name to the Long Mead End species, it 

 may well be called after our most diligent explorer of Eocene 

 Mollusca. 



The valves are unequal, right valve considerably overlapping 

 the other, both the posterior and low borders. The left or smaller 

 valve is slightly angulated with a posterior area, but the feature is 

 more obscure in the other valve. The shell is rather smooth, but 

 with delicate growth-striae. It is very much flatter and smaller 

 than G. striata ; the outline is not unlike that of G. oblong a, Desh., 

 but it is less inequilateral ; the umbos are nearly median. Length 

 \ inch, by rather less than T 3 g breadth. 



Gorbula cuspidata, Sow. We have this both from the Long 

 Mead Mead End sands and the Beacon Bunny clay below, it 

 occurs lower down in the Barton and above in the Headon beds. 



[C. fortisulcata, Edw. MS. There is a shell so determined in 

 the Edwards collection as from Long Mead End ; it is however 

 from clay, and not from the Upper Bagshot sands : it is perhaps 

 only a variety of C. pisum, Sow.] 



[Gkama squamosa, Brand. It is said on the authority of 

 Dr T. Wright, in Lyell's Student's Elements of Geology, p. 233, 

 that this shell occurs in the Upper Bagshot sands. This is 

 apparently an error ; the specimen must have come from Barton 

 sandy beds.] 



(3) On some equations connected with the Electromagnetic 

 Theory of Light. By R. T. Glazebrook, M.A. 



The problem of the reflexion and refraction of Light on 

 Maxwell's Theory has been treated by Lorentz (Schlo milch Zeit- 

 schrift, Vol. xxii.), and again by J. J. Thomson, (Phil. Mug., 

 April, 1880), so far at least as isotropic media are concerned, 

 and also by Fitzgerald (Phil. Trans. 1881). The following paper 

 gives a method of obtaining the intensities of the reflected and 

 refracted plane waves in the most general case, that of a wave 

 travelling in a crystalline medium incident on the surface of 

 another crystal, which is more simple than those of Lorentz or 



