298 ft. MORLAND ON DIATOM STRUCTURE. 



but if the ends of these borders be examined it will be found that 

 they join each other, provided the fine adjustment be carefully 

 worked whilst they are under examination. But in addition to 

 what I have just stated, I can add that I have a slide of Pleuro- 

 sigma Balticum in which one of the valves, in consequence of a 

 side fracture near one of the ends, is split right up the centre of 

 the median line from one of the end nodules to the centre one. 

 Now, it must be evident that if the median line, or " raphe," were 

 a simple thickening of the valvular structure, it would be the last 

 place where a fracture could occur along its length. . Beyond all 

 this, I have a valve of a Navicula which I happened to 

 slightly fracture whilst washing it free from adherent dirt in a 

 drop of water before placing it into position in a " selected " 

 mount. It got fractured near one of the end nodules, and this 

 fracture ran right along the whole length of the valve, through 

 the centre nodule to the one at the other end. At the moment 

 when I touched it with my mounted bristle it opened out just like 

 a pair of scissors, but on removing the bristle, as the other end 

 still remained intact, the two halves sprang back again into their 

 original positions, and under a low power the valve still looks 

 perfect. I have mounted this valve by the side of an unbroken 

 one, in which the centre nodule is seen to be perfect between the 

 two central ends of the two halves of the " raphe," whilst in the 

 broken valve the two halves of the "raphe" are seen to join each 

 other right across the centre nodule. It is not at all an un- 

 common thing to come across valves of Navicidce halved along the 

 " raphe " when looking over ordinary u spread " diatom slides. 



Last summer, being in Jutland, I obtained a quantity of the 

 well-known Jutland " Cementstein," from both the islands of 

 Mors, and Fur. As I had already had this material in my hands 

 for some time past, it was, so far as cleaning it for diatoms in the 

 ordinary way was concerned, of little or no use to me ; but 

 noticing under a Coddington lens that, as a general rule, the larger 

 Coscinodisci lay parallel to the stratification, I determined to pre- 

 pare sections in which I could cut these Coscinodisci in any direc- 

 tion I chose. 1 accordingly sent some pieces of this material to 

 a lapidary for slicing — first marking with ink the direction in 

 which the material was to be cut up. I may here remark that this 

 " Cementstein " is simply perfection so far as regards texture and 

 hardness. With care, sections can be prepared of exceeding 



