84 L. Trouvelot—Undulations in the tail of Coggia’s Comet. 
exception of Strongylocentrotus Gaimardi A. Agassiz, have been 
found by the Geological Commission. The localities at which 
they were obtained are so nearly those recorded by Mr. Alex. 
Agassiz in his Revision of this group, that it would be of no 
value to enumerate them again here. ‘The species are as follows: 
Cidaris tribuloides B1., Hipponoé esculenta A. Agassiz, 
Arbacia pustulosa Gray, Clypeaster subdepressus L. Agassiz, 
Diadema setosum Gray, HMellita testudinata Klein, 
Echinometra subangularis Desml., Mellita sexforis A. 
Agassiz, 
Strongylocentrotus Gaimardi A. Agassiz, Encope emarginata L. Agassiz. 
Toxopneustes variegatus A. Agassiz, 
It has been noticed that Mellita testudinata is brilliantly phos- 
phorescent at times, when it has been removed from the water 
or a few hours, though it be still wet and apparently alive. 
The phosphorence is very apparent over the entire body. I 
indebted to Mr. Alex. Agassiz for kindly examining and veri- 
fying the determinations of a collection of the above Hchint 
sent to the United States, excepting Mellita sexforis, which has 
been found only very recently. 
Art. X.—Undulations observed in the tail of Coggia’s Comet, 
1874; by L. TrRovuvetor. 
On the evening of July 21, 1874, at 95 0™, the moon being 
in her first quarter, and the sky remarkably clear even close to 
the horizon, my attention was attracted by a bright ray of light 
darting from the northwestern horizon, way up in the constel- 
lations. Taking it for an auroral phenomenon, I went in for 
the spectroscope ; but on my return, after a few seconds, to my 
disappointment I found no more trace of it. Soon, however, 
it reappeared, and darted up in an instant after the manner of 
certain auroral rays, and vanished again after ten or fifteen 
seconds. I then Scenis aware of my error, and found with 
surprise that the phenomenon was taking place in the tail of 
Joggia’s Comet, the head of which was then plunged under 
the horizon. 
During the whole time that I observed this interesting phe- 
nomenon, I saw the comet’s tail shortening and extending, 
lightning up and extinguishing like the rays of certain auroras. 
Extended undulations, rapid vibrations, ran along it in succes- 
sion from the horizon to its extremity, giving it the appearance 
of a fine gauze wavering in a strong breeze. The. pulsations 
and the waves of light were of unequal duration; some being 
rapid, while others lasted a longer time. For over one hour 
the comet's tail kindled and extinguished more than one hun- 
dred times; the extinction being sometimes so complete that 
