REVIEWS AND BOOK NOTICES. 225 
all the principal museums in Europe as well as our own country, it 
must for many years be the standard work on this subject. 
The first part contains the bibliography, followed by a chapter on 
“ Nomenclature, which will greatly interest special students in zo- 
ology, and we only wish we had room to reprint certain portions. 
Mr. Agassiz finds that the value of the genera usually recognized, 
“ when tested by our present knowledge of the changes they un- 
dergo seems limited almost to convenient headings or keys for the 
more ready identification of species. Genera, as we recognize 
them among Echini, are certainly not founded upon features of gen- 
eral and permanent value, but, on the contrary, upon features 
applying only to a few species, and of very limited application.” 
“ In spite of the definite existence of what we call species, genera, 
etc., when we apply these terms to limited regions and series of 
the present day, yet we find them totally inadequate to express our 
wider interpretation when our standards of comparison are infinite 
in time or space.” In illustration of this idea expressed of the 
shifting nature of generic characters, if we understand our author 
aright, he says in the preface to his second part that “ the number 
of fossil genera has been increased to such an extent, and they 
haye been based upon features which are here shown [by the study 
of the young of existing species] to have so little value, that be- 
fore we can make a satisfactory comparison of the fossil species 
with those now living, a thorough reéxamination of se ossil 
Echini from our present standpoint is absolutely necessary.” 
The ‘chronological list” contains the history of the names of 
the different forms of Echini; 116 pages are devoted to synonymy. 
An interesting chapter on “ Geographical Distribution,” ends part 
I. In it he advocates “ Lovén’s theory of the distribution of life, 
mofla mora a throughout the bottom of the deeper parts 
of the Atlantic. 
Mr. Agassiz thinks that we now have a very fair representation 
of the littoral Echini of the world, and as recent explorations 
indicate that we have hitherto inadequately mapped out the prob- 
able distribution of life at great depths, he would wait for the 
results of such explorations before discussing the subject. He 
finds that the distribution of sea urchins agrees remarkably with 
_ the “ great belts of temperature first mapped out by Dana ;” and 
copies them on the seven suggestive maps bound in before the 
es, 
AMER. NATURALIST, VOL VIL 15 
