20 GEOLOGICAL MEMOIRS. 



in the central zone. But the 161 species represent the contributions 

 of fifteen countries, which consequently possess each, on an average, 

 10-73 species. On the other hand, the 80 species of the central 

 zone, coming only from four countries, give to them an average of 

 20 species, or nearly double the proportion of those from the 

 northern zone. 



M. Barrande, however, states with regard to these results that 

 they are principally due to the introduction of the contingent which 

 Bohemia has furnished to the palaeontological enumeration, and 

 may therefore be modified or reversed by discoveries in other 

 countries at present unexplored. 



The Pteropods, hke the Trilobites, tend to confirm the line of de- 

 marcation between the Palaeozoic and Mesozoic eras, and by the 

 distinctive characters of their specific and generic forms furnish a 

 ready means of recognizing the principal stratigraphical divisions of 

 the different systems. 



In conclusion, M. Barrande caUs attention (1) to the remarkable 

 contrast between the great numbers of the most ancient representa- 

 tives of this order in the Silurian seas (that is to say, soon after their 

 first appearance) and the small number of species which now exist, 

 and (2) to their extraordinary disappearance during the whole of 

 the Mesozoic era, although they reappear in the Tertiary formation 

 and have in the existing fauna certain forms very analogous to those 

 from the Palaeozoic series. 



If this intermittence be only apparent, and attributable to the 

 exclusive existence of species unprovided with a shell during this 

 immense lapse of time, it will not be less interesting to inquire why 

 the order of Pteropods is the only one which, during this long 

 " struggle for existence," successively carried, resigned, and re- 

 sumed its testaceous covering. [A. S.] 



On the Eruption at Santoeino. By Chev. von Hauee. 



[Proceed. Imp. Geol. Instit. Vienna, May 15, 1866.] 



Prof. Zephaeovich has recorded the occurrence of crystallized horn- 

 blende in a specimen from the new centre of eruption on George I. 

 Point. Dr. Stache has minutely examined the felspathic sub- 

 stance included in the cellular interstices of these lavas, its specific 

 gravity being 2-66. Analyses of the following varieties have been 

 made in the laboratory of the Imperial Geological Institute : — A. 

 From the old crater of ISTea Xaimeni ; greyish-black, with fine pores ; 

 the main substance grey, with isolated small crystals of felspar, mag- 

 netic oxide of iron, and alternatiiDg grey and black stripes, according 

 to the admixture of more or less felspar. B. Prom the banks of the 

 freshwater lakes of Nea Kaimeni, behind the bathing-houses ; black 

 pitchstone-like lava, with traces of parallel lamellae and a very 

 small amount of disseminated white vitreous felspar. C. Prom the 

 slope beneath Thera (Santorino), close to the level of the sea ; black 

 cellular obsidian slag, with traces of parallel structure. 



