:}r>,s The American Geologist. May, 1894 
characters of shagreen, and probable disposition of paired and unpaired 
fins." 
'/'//< La Plata Museum. By R. Lydekker, B. A.. P. Z. S. In Natur- 
al Seiene{ (vol. iv. Nos. 23, 24) Mr. Lydekker gives an interesting ac- 
counl of i his Museum, so rich in fossil vertebrates. He describes man) 
of the best examples from the Pampean, Patagonian, and other deposits. 
He Finds thai very mam of the new genera (named "by certain South 
American paleontologists) are but speciesof previously described genera, 
or are the same genera wilh different names. The author well de- 
scribes this surprising contusion when he says: "Indeed, on the princi- 
ple tor rather, warn of principle) which appears to have guided the Ar- 
gentine paleontologists, about a dozen species and some half a dozen 
genera mighl easily be made out of the remains of the common horse." 
Tli is statement is the outcome of his examination of some of the forms 
with dilferent generic names in which the dentition had assumed dif- 
ferent forms according to the age of the individual in question, and he 
adds, "which even any ordinary student would say were identical." 
After reading Mr. Lydekker's account of the wonderful treasures in 
this museum, one regrets that these fossils are not more accessible; for. 
with what has been remarked above, it is almost unnecessary to say 
that these descriptions bj South American workers are not strictlj re- 
liable. 
Tin Mean Density of tin Earth, an essay to which the Adams prize 
was adjudged in I893in the University of Cambridge. By J. II. Poyx- 
tixo. 8vo, pp. \x. 156, wilh illustrations and seven folding plates. (Lon- 
don, ('has. Griffin & Co.. 1894.) In this work we rind a historical ac- 
count of the various attempts to determine the mean density of the 
earth from the time of Newton to the present. "It is not a little re- 
markable that Newton hit upon the limits between which the values 
found by subsequent researches have nearly all lain."* The author de- 
scribes some experiments of his own, made with the common balance, 
in which he obtained the figure 5.493 as the earth's average density. 
The observations and mathematical calculations are given with consid- 
erable detail, and the various factors of the problem are elaborated full) . 
The average of the fourteen dilferent determinations that have been 
completed by different calculators gives the figure 5.523. The work 
shows ureal care in preparation, and furnishes a complete resumed of the 
subject, thus constituting a valuable work for reference. H. Y. W. 
On tin chemical composition of ehondrodite, liumiU mnt clinohumite. By 
s. L. Pkm'iki.d and W. T. H. Howe. Amer. .lour. Sci., vol. 47. pp. 188 
206, March. 1894. By a series of careful analyses and a comparison 
with Other analyses the authors show that these three minerals are not 
identical in chemical composition. The formula' established are: 
Chondrodite Mg 3 |Mg(F. OH)] 2 [Si04]2 
llumite Mg 6 |MgiF. OH)] 2 [8i0 4 ] 3 
Clinohumite Mg 7 | Mg (F. OH)] 2 [8i0 4 ]4 
The\ thus are seen to form a series which varies progressive!) from 
