We wish the authors could have given us some infor- 
mation as to the modus operindi of the Teredo in exca- 
vating its cylindrical tube, instead of merely quoting 
Kater’s opinion that the shell is the boring organ. One 
thing is certain, and indeed has been admitted by Kater, 
that the foot of Zeredo is in front, occupying the bottom 
of the tube, while the shell at the same time occupies that 
part of the tube which lies immediately above the foot, 
and is closely pressed against the sides of the tube. To 
suppose that the position of the foot and shell could be 
reversed by the animal, so as to make the shell lie at the 
bottom of the tube and the foot on one side during the 
process of excavation, is quite inconsistent with our 
knowledge of the Zervedo and of the habits of other 
boring and burrowing Mollusca.  Solen, \ Cardium, 
Natica, Acteon, and many other kinds burrow in sand 
by means of their strong muscular foot; Pholas 
dactylus occasionally does the same ; and the limpet uses 
its foot only for excavating the hard rock in which it is 
sometimes more or less deeply imbedded. The gradual 
enlargement throughout of the tube of Zeredo, especially 
at the opening (where the siphons are placed), cannot 
possibly be caused by the shell, which invariably lies at 
the other end; and the prickles which cover the surface 
of the shell, and enable it to act as a fulcrum or Joint 
d’appui, could not be renewed if they were continually 
employed in rubbing away the wood. There can scarcely 
be a question that the foot is the sole instrument of per- 
foration in Zeredo, as it is in Solen, Pholas, and Patel/a. 
J. Gwyn JEFFREYS 
OUR BOOK SHELF 
The Student's Manual of Comparative Anatomy and 
Guide to Dissection. Part I. (Mammalia). By G. H. 
Morrell, M.A. (Longman aad Co.) 
THIS work is in two parts, which are of such different 
characters that they must be considered separately. The 
first is intended to include a short and complete summary 
of the main facts of the anatomy of Mammalia. This is 
a large undertaking, and one which a resident in Oxford 
has not full opportunities of completing ; for the ad- 
vantages in any place other than London, are not 
sufficient to enable any single student, however enthusi- 
astic, to get familiar with many of the subjects discussed. 
There is a want of vividness and point in many of the 
statements, several of which are too inclusive. Referring 
to the lobulation of the kidneys, the seals and whales are 
mentioned as presenting it, but why are the ox, otter, and 
rhinoceros omitted? The peculiarity of the stomach of 
the chevrotain is not referred to, and all we can possibly 
infer as to that of the peccary or hippopotamus is that it 
is constricted into two or three portions, which is un- 
doubtedly not enough. Half a page only is devoted to 
the peculiarities of the liver throughout the class, and that 
of man is called simple, while that of the Ruminants is 
included among the multifid. The spleen of the mar- 
supials is stated erroneously to be bent or bilobed. 
But the great and inexcusable imperfection of the work 
is the omission of the description of the generative system, 
which no amount of argument could persuade us will 
prove of the slightest good in any way. It only engenders 
a mystery and curiosity in the mind of the younger 
students, as to peculiarities of structure, which if they 
‘were treated in the ordinary routine, would, as they un- 
doubtedly are among medical students, be looked upon 
in nothing but a common-place manner. 
The second portion of the work, the guide to the dis- 
NATURE 
aa 
Or +5. : 
[May 1, 18 
sections of the brain, heart, &c., of the sheep are excel- 
lent, and will be found of great value ; they have long 
been wanted by teachers. A carefully compiled synopsis 
of the cerebral convolutions in man and the higher apes, 
from the work of M. Gratiolet, terminates the book. 
J 
Centiéme Anniversaire 
Académie Royale de Belgique. 
(Brussels; F. Hayez, 
de Fondation. Two vols. 
1872.) 
THESE two stout volumes, intended as a memoria 
of the celebration of the hundredth anniversary of 
the Belgian Academy, treat of a great variety of 
interesting and valuable matters. The Belgian Academy 
of Science, Literature, and Art was founded by Maria 
Theresa on December 16, 1772, but as December is not a 
very suitable month for a great public gathering of men 
from all parts of Europe, the Academy held its centenary 
féte on May 28 and 29, 1872, and it did it very royally, in 
presence on both days of His Majesty the King of the 
Belgians, who gave the opening address, and entertained 
members and friends on the second day in his palace at 
Brussels, There took part in the celebration distin- 
guished deputies from all the countries of Europe 
and from America, and altogether it seems to have 
been a great success. In these volumes will be found 
a detailed account of all that was said and done, 
verbatim reports of all the speeches made, and of all the 
interesting papers read. The Academy began to make 
preparations for the centenary celebration in 1869 by the 
appointment of a commission. This commission ap- 
pointed members of the various classes of science, litera- 
ture and art to prepare papers giving accounts of the 
work done in these classes from the commencement, and 
others to do the same for the various literary, antiquarian, 
artistic and scientific subjects with which the Academy 
deals. From this it may be surmised that these two 
volumes contain matter of very great value indeed. The 
first paper is by M. A. Quetelet, giving a sketch (170 
pages) of the history of the first century of the Academy. 
But the second volume will be the more interesting of the 
two to scientific men ; we can only indicate its contents :— 
Astronomy in the Royal Academy of Belgium from 1772 
to 1872, by M. E. Mailly ; Report on the Mathematical 
works of the Academy during the same period, by M. J. 
M. de Tilly ; Report on works in the Physical Sciences, 
Meteoro'ogy, and Physical Geography, by M. J. Duprez ; 
Report on works in Chemistry, by M. L. G, de Koninck ; 
Report on works in Zoology, by P, J.van Beneden ; Report 
on works in Botany and Vegetable Physiology, by M. E. 
Morren ; Repo-t on works in Geology and Mineralogy, by 
M. G. Dewalque. 
LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his correspondents, No notice is taken of anonymous 
communications. | 
Biela’s Comets 
THE present note is designed to show that several comets 
move in nearly the same orbit with that of Biela; that they 
probably entered the solar system asa group; and that, after 
making their first perihelion passage in close proximity to each 
other, they were, when receding from the sun, thrown into 
their present orbits by the disturbing influence of Jupiter. 
1. Was the comet of 1772 identical with that of Biela?— 
The mean of theseven consecutive periods between January 2, 
1806, and September 23, 1852, is 2437°7 days. Counting five 
periods of the same mean length from February 17, 1772, brings 
us to July 2, 1805—six months before the perihelion passage 
of 1806, In other words, the mean period between 1772 and 
1806 was greater by about thirty-seven days than that between 
1806 and 1852. The perturbations during the half century 
succeeding the apparition of 1772 have not been computed. It 
seems very unlikely, however, that the difference of periods 
