JAEKEL ON MICRO-STRUCTURE. 169 



Logic state of preservation of the remains, it unfortunately leaves much 

 to be desired. In a general way. the fossils show merely the coarser 

 histologic structure, while the finer details are for the most part invisible. 

 The material is in this respect somewhat in the same condition as the 

 Devonian fish remains from the Old Red sandstone of Scotland, in which 

 likewise the finer histologic details are usually not present, while in the 

 remains from the Russian Devonian they are finely preserved. The state 

 of preservation depends on the retention of the fine dentine and primi- 

 tive tubules ; and this again depends on their being tilled with air or 

 with a dark infiltrate. At times it is seen that in one part of the slides 

 the fine canals are completely preserved, while in the other parts of the 

 same preparation either (a) only single parts of the tubules are preserved 

 or (h) the tubules are altogether invisible. In such case the outlines of 

 the tubules are sometimes seen in oblique illumination. This is the case 

 with our fish remains. The fine details are mostly invisible, hut are 

 preserved in some parts and may then he easily recognized with an 

 oblique converging light. Add to this that all hard parts are more or 

 Less worn and probably changed in various ways by acids. This being 

 premised, the micro-structure exhibits the following conditions: 



Figure 1 of plate 5 shows a vertical section through a scale or a cara- 

 pace fragment. In the upper part of the preparation there are seen 

 tubercles of dentine ( f)), containing a pulp'from which numerous den- 

 tine tubules run out. These are especially well preserved in part in the 

 middle dentine tubercle, while the outlines of the pulp appear greatly 

 corroded. These conditions are seen more distinctly in figure 2, in which 

 two dentine tubercles lying side by side are enlarged aboul 7'* diameters. 

 Here not only are the dentine tubules seen well preserved, hut the out- 

 line of the pulp, too, is unchanged. It is furthermore important to note 

 in them the concentric lamination, which appeal's in primary connection 

 with the dentine tubules. The concentric lamellae do not run in uniform 

 curves, hut arch independently between the dentine tubules, the curva- 

 ture being directly inward. Toward the outside the Lamellae run more 

 uniformly parallel to the surface. This concentric building up out of 

 lamellae appears with like distinctness in the dentine tubercle represented 

 in figure 3, which in its outer form reminds one of a tooth. It also 

 greatly recalls the teeth which are described by Rohon from the blue clay 

 of St. Petersburg. There can hardly he any doubt that this concentric 

 structure of the hard parts represent- a Low stage of development. At 

 any rate. I believe that the most essential difference between the calcified 

 hard part- of the lower animals and those of the vertebrates consists in 

 this: that in the former growth took place only by apposition, and that 



XXII- !'•< m Gkoi 3oi \ .i . \ -i 3, 1891. 



