Correspondence. 189 
selves destined to be modified by similar causes. The known per- 
manence of certain animals and plants for several hundred years, and 
the sterility of hybrids, were adduced as evidence that aberrant 
animals and plants recur to the type from which they have diverged, 
and that true species are permanent. 
The Chairman, alluding to the admitted progressive character of 
fossil species, wished that the Lecturer had been able to take into 
consideration the cause of that progressive advance. 
CORRESPONDENCE. 
Sua 
Note oN BELEMNITES CLAVATUS, BLainv.—I am happy to be 
able to reply to the inquiry of Professor Phillips, regarding the 
form of the phragmacone in Belemnites pistilliformis, Sow. (B. cla- 
vatus, Blainv.), by the statement that it has proportions similar to 
those of other Belemnites, and is not slender and elongated as in 
Xiphoteuthis. A specimen showing this (a guard and phragmacone, 
in stone, rubbed down and polished so as to exhibit a section) was 
lately in my collection, and is now, if I mistake not, in the Museum 
of Practical Geology. 
Note ON XIPHOTEUTHIS ELONGATA.— Since the publication of 
Professor Huxley’s Memoir, I have obtained a fine specimen (now in 
the Jermyn Street Collection) of this species. ‘The guard of this 
example was fractured in extraction from the matrix, and showed 
clearly that, in this instance at least, it was tubular throughout, 
though closed ai the extremity. From what I have seen in other 
specimens -that have come under my notice, I am strongly of opinion 
that this is a normal character, and that a solid guard is the result 
of fossilization.— Epwarp C. Harrsinck Day, Charmouth. 
SuURFACE-MARKINGS ON SANDSTONE. 
To the Editor of the GEOLOGICAL MAGAZINE. 
Srr,—For many years the peculiar markings found so frequently 
in sandstone and other rocks, from the Cambrian upwards, have 
been attributed by Geologists to the effect of heavy rain-drops falling 
on plastic mud, or moist impressible sand. ‘Together with my fel- 
lows, I believed the markings to be due to rain-drops, until a fortu- 
nate discovery proved the conjecture untenable. During the summer 
of 1859, while shooting sea-fowl, shortly after sun-rise, on the sands 
near Leith, I observed peculiar depressions, with cusps, precisely 
similar to those so well represented in Plate IV. in No. IX. of the 
GroLocicaL Macazine. ‘They were identical with the depressions 
caused by rain-drops during a thunder-storm; butas the streets in the 
town were dry, my curiosity was excited. On calling the attention 
of a neighbouring watchman to the markings, he informed me that 
no rain had fallen through the night, and so I looked for another cause 
