AT THE MICROSCOPE. 37 



increasingly recognised. He has shown (and his observations 

 may be readily confirmed and extended) that certain natural 

 orders are characterised by the presence of special types of crys- 

 talline matters, whilst others of these great divisions are equally 

 conspicuous by their absence. There are, however, a few genera 

 possessed of them amongst orders which (with these exceptions) 

 have them not. Mercurialis is an example of this remark, and 

 a very interesting one. 



The EuPHORBiACEiE being mostly free from raphides ; but 

 their nearest natural order is that of the TjRTiCACEiE, which are 

 eminently raphidiferous (using the term in its widest signification). 

 Now, Herb Mercury, though the fructification is truly Euphorbia- 

 ceous, yet undoubtedly has the habit of a nettle. Possessing such 

 habit, and abundant " sph^raphides " as well, it forms evidently a 

 connecting link between the two orders. It will be desirable to 

 examine the composition of these crystals. In some similar instances 

 the larger have been found to be carbonate, the smaller oxalate of 

 lime. The gradual alteration of the crude sap by abstraction of 

 mineral matters in the course of circulation is beautifully seen here. 

 It will be observed that there are multitudes of Sphaeraphides 

 present in the midrib, along which the sap rushes after coursing up 

 the stem, to the marginal parenchyma of the leaves. Some of the 

 veins, in the earlier part of their course^ also show raphides, but 

 these gradually become fewer, and presently the veins are seen to 

 be entirely free from them — thus showing clearly how the mineral 

 matters have been successively parted with, to combine with al- 

 buminous material, for which they have a great affinity. 



Cowrie shell (Plate 8, upper half). — In the transverse section 

 of Cowrie, it is impossible not to be struck with the general resem- 

 blance in the structures present to dentinal tissue. Both externally 

 and internally is a hard, glassy layer, formed of prisms placed at 

 right angles to the varying surfaces, which may be compared to 

 enamel. The intervening coloured portion is laminated. At the 

 thickened bases of the hollowed cone (to which the shell in its 

 general form may be compared), the structure closely resembles 

 that of the dentinal tubuli, or the shell of some of the larger 

 Crustaceans. 



In the arched vault of the shell interlacing fibres are seen more 

 or less distinctly, whose general direction is at an angle of 45 '^ to 

 the surface. This appears to be due to a secondary arrangement 

 of the crystalline particles of the shell. The effect of such disposi- 

 tion must be enormously to increase the strength of the materials 

 entering into its composition. In connection with such structural 

 arrangement, it is interesting to recall the interlacing fibres found in 



