202 PEOF. AV. J. SOLLAS OX SACCAMMIXA CARTEET, [vol. Ixxvii, 



63-i° C), examined them under the microscope, and was then able 

 to observe a g-lobulitic structure which he believed to be original ; 

 and he considered that the interspaces between the glohulites were 

 originalh^ occupied by the keratin network previoush^ described. 



Since calcium carbonate begins to decompose below 634° C. 

 these conclusions seemed to be doubtful. I therefore prepared a 

 thin slice of OrhitoUfes, and, after making sure that it clearly 

 displa^^ed the characteristic fibrillar structure, heated it for half a 

 minute in molten potassium iodide. It was then freed from the 

 potassium iodide, mounted in balsam and examined under the 

 microscope. The globulitic structure was fully displaj^ed, but all 

 traces of the original fibrillar structure had disappeared. This is 

 suggestive of an artefact oi'igin. On the other hand, attempts at 

 measurement of the globulites on the one hand and the granules 

 present in the original structure on the other showed a rather 

 close approximation in size, the granules being slightly the 

 smaller. 



Exact measurement was impossible : in the first place the divi- 

 sions in my eyepiece-micrometer were too widely spaced, and in 

 the next it was impossible to determine precisely the boundary of 

 the object. One division of the eyepiece scale corresjDonded to a 

 length of 0"0023 mm., and the diameter of the globulites was 

 estimated as about half of this, sometimes more, sometimes 

 less. On the dark field produced by crossed nicols, however, the 

 outlines of both granules and spicules is sharply defined, and their 

 apparent diam.eter could be precisely measured. It was estimated 

 as one-fifth of a division, or 0'000-16 mm., thus corresponding with 

 the wave-length of blue light. This is well within the limit of 

 resolvability (0"00027) but beyond the theoretical limit of visi- 

 bility. It may be a diffraction effect, as Prof. Lindemann points 

 out ; but, even so, it indicates the existence of such particles as 

 theory requires. 



The light transmitted b}^ the shell is not a pure yellow, however, 

 but tinged with brown ; and Prof. Lindemann suggests in explana- 

 tion of this that a certain amount of white light may be reflected 

 by the larger granules, whence some resulting blackness, which, 

 added to the yellow, produces a brown shade. 



It now only remains to determine, with the aid of a c[uartz- 

 wedge or a selenite-plate, the direction of the optic axes in relation 

 to the fibrillse and their disposition in the substance of the shell. 

 Numerous observations show that in some cases the optic axis is 

 coincident with the length of a fibril, but in others it is as 

 definitely transverse. 



The transverse direction is scarcely what we should expect in an 

 elongated crystal of calcite, and suggests a comparison with that 

 variety of this mineral which has been named ' lublinite.' This 

 occurs as felt-like inters-rowths of minute acicular crystals (not 

 exceeding 0'02 mm. in length) with very oblique extinction, which 

 Quercigh regards as rhombohedral crystals greatly elongated 



