
Oct. 19, 1871] 
of ossicles ; or, in rare cases (Amphoracrinus), tunnelled in the 
substance of the greatly thickened walls of the vault. These 
internal passages, usually reduced in number to five by uniting 
with one another, pass into the internal mouth, into which 
they doubtless lead the current from the ciliated brachial 
grooves. 
[n connection with different species of /atyceras with various 
crinoids, over whose anal openings they fix themselves, moulding 
the edges of their shells to the form of shell of the crinoid, is a 
case of ‘‘commensalism,” in which the molluse takes advantage 
for nutrition and respiration of the current passing through the 
alimentary canal of the echinoderm., //yfonome Sarsii appears, 
from Prof, Lovén’s description, to be a true crinoid, closely allied 
to Antedon, and does not seem in any way to resemble the Cysti- 
deans. It has, however, precisely the same arrangement as to 
its internal radial vessels and mouth which we find in the older 
crinoids. It bears the same structural relation to Anéedon which 
Lixtracrinus bears to Pentacrinus, 
Some examples of different tesselated crinoids from the Burling- 
ton limestone, most of them procured by Mr. Wachsmuth, and 
described by Messrs. Meek and Worthen, show a very remark- 
able convoluted plate, somewhat in form like the shell of a 
Scaphander, placed vertically in the centre of the cup, in the 
position occupied by the fibrous axis or columella in entacrinus 
and Antedon, Mr, Billings, the distinguished paleontologist to 
the Survey of Canada, in a very valuable paper on the structure 
of the Crinoidea, Cystidea, and Blastoidea (Si//iman’s Journal, 
January 1870), advocates the view that the plate is connected 
with the apparatus of respiration, and that it is homologous with 
the pectinated rhombs of Cystideans, the tube apparatus of Pen- 
tremites, and the sand-canal of Asterids. Messrs. Meek and 
Worthen and Dr, Liitken, on the other hand, regard it as asso- 
ciated in some way with the alimentary canal and the function of 
nutrition. 
The author strongly supported the latter opinion. The peri- 
visceral membrane in Anéfedox and Pentacrinus already alluded 
to, which lines the whole calyx, and whose involutions, support- 
ing the coils of the alimentary canal, contribute to the formation 
of the central columella, is crowded with miliary grains and 
small plates of carbonate of line ; and a very slight modification 
would convert the whole into a delicate fenestrated calcareous 
plate. Some of the specimens in Mr. Wachsmuth’s collection 
show the open reticulated tissue of the central coil continuous 
over the whole of the interior of the calyx, and rising on the 
walls of the vault, thus following almost exactly the course of 
the perivisceral membrane in the recent forms. In all likelihood, 
therefore, the internal calcareous network in the crinoids, whether 
rising into a convoluted plate or lining the cavity of the crinoid 
head, is simply a calcified condition of the perivisceral sac. 
The author was inclined to agree with Mr. Rofe and Mr. Bil- 
lings in attributing the functions of respiration to the pectinated 
rhombs of the Cystideans and the tube apparatus of the Blastoids, 
He did not see, however, that any equivalent arrangement was 
either necessary or probable in the crinoids with expanded arms, 
in which the provisions for respiration, in the form of tubular 
tentacles and respiratory films and lobes over the whole extent of 
the arms and pinnules, are so elaborate and complete. 

ON THE RELATION OF AURORAS TO 
GRAVITATING CURRENTS * 
ROF. LOOMIS’S observations of the number of Auroras in 
each month of 1869 and 1870 (American Fournal of Science, 
3rd S., i. 309) are specially noteworthy, both because of the 
careful accuracy of the observer, and because they are the first 
published observations which furnish satisfactory data for an ap- 
proximate determination of the Jaws of auroral distribution. 
If the auroras are, as is now generally believed, luminous 
manifestations of terrestrial magnetism, it seems reasonable to 
look to them for some additional evidence upon the question of 
the relation between magnetic and gravitating currents. Messrs. 
Baxendeli and Bloxam have already pointed out some resem- 
blances between hyetal and magnetic curves (see Proc, A. P. S., 
x. 368) and if analogous resemblances can be traced between 
hyetal and auroral curves, they will be interesting and suggestive. 
I have not found the similarity between the annual distribu- 
tion of rainfalls and auroras sufficiently striking to impress any 
* Read before the American Philosophical Society, May 5, 1871, by Plin 
Earle Chase. ; 4 f 
NATURE 


497 

one who has not made a special study of the causes of resem- 
blance and difference. But, as Ihave repeatedly urged, currents 
are subject to an increased number of disguising disturbances, in 
proportion to the sluggishness of their motion, and the time 
which is consequently required for their formation and change. 
We may very reasonably look for analogies between the daily 
and the anoual auroral or magnetic curves, of a character for 
which we could hope to find no parallel in wind, rain, or ocean 
current curves, 
If we desire, therefore, to find evidence of the joint influence 
of solar expansion and gravitating equilibrium, we should look 
where it is most likely to be found, and to the best of the obser- 
vations which may be supposed to be fairly comparable. There 
are similar variations of solar attitude, and consequently increas- 
ing and diminishing solar force, in the day and in the year, but 
the effects of these variations upon the precipitation of vapour are 
more likely to be shown in their greatest simplicity by the means 
of observations at different hours of the day than at different 
seasons of the year. I know of no published observations of 
this character at New Haven, but there are some extending over 
a long series of years at Philadelphia and at Greenwich, the 
curves at each station indicating minima of rainfall at noon and 
midnight, and maxima in the morning andevening. The differ- 
ence of longitude between Philadelphia and New Haven being 
less than 24", it is not likely that there is any material difference 
in the daily rain-curves at the two places. 

In order to make the curves fairly comparable, both in regard 
to the times and the magnitudes of deviation, I treated the auro- 
ral observations in the same manner as those of rainfall (Proce. 
A. P.S, x. 526). Both in the magnetic and in the hyetal phe- 
nomena, the greatest effects accompany the grea'est atmospheric 
changes. But inthe magnetic disturbances the principal maxima 
occur in the spring of the year and the morning of the day, while 
the general evaporation is increasing ; whereas, in the daily rains 
at Philadelphia, the principal maximum occurs in the afternoon, 
when evaporation is diminishing. I have, therefore, compared 
the midwinter ordinate of the auroral with the noon ordinate of 
the rain curve, and the midsummer auroral with the midnight 
hyetal ordinate. 
The auroral observations and the normal ordinates of the ac- 
companying curves are given in the following table. I presume 
no one will doubt that the condensation of vapour, which is re- 
presented by the rain-curve, is occasioned by the simple operation 
of gravitation in blending currents of different temperatures, 
and I see no reason for postulating any different law for the de- 
velopment of electricity and magnetism in the aurora, 
Comparative Table of Auroras and Rainfalls 
Month. ecaote Normals. Hours. pote 
88 fo) gL 
January ; 5 90 I ou 
94 2 93 
February 3 E31 98 3 98 
103 4 105 
March . 2 AT 107 5 110 
109 6 113 
April, : 44 109 7 113 
108 8 112 
