Juxy 16, 1897.] 
life to be preserved in the fossil state, represent- 
ing, as Saporta and Marion show, and as Mr. 
Tyler’s researches fully bear out, the primitive 
form of leaf development, which consists simply 
in setting apart a portion of the growing plant 
to serve the purposes of leaves, consisting of 
more or less broad and elongated blades, usu- 
ally embracing the stem and tapering gradu- 
ally to a point, with the leaf bundles continued 
in straight lines parallel to each other through- 
out their entire length. They are, therefore, 
broadest at the base and least adapted to secur- 
ing the ultimate purpose of leaves already men- 
tioned, viz., the maximum amount of light and 
air. The process of leaf development began 
with this condition, and many of the forms in 
which the cotyledon is still single have acquired 
a blade, as, for example, many species of Pota- 
mogeton, Smilax, Dioscorea, etc. In Smilax 
and some other genera true stipules have been 
developed, along with the tendency towards 
their differentiation into tendrils and other use- 
ful organs. 
An important obstacle to the preservation of 
monocotyledonous leaves in the fossil state is 
the absence in them of any definite joint or 
natural point of separation of the leaf from the 
stem, which is one of the earliest results in the 
process of leaf evolution, also involving the 
principle of the renewal of leaves at annual or 
other fixed periods, which has practically re- 
sulted in the indefinite multiplication of the 
leaves produced, increasing the chances of their 
preservation by the whole number of such re- 
newals. The only chance for an ordinary mono- 
cotyledonous plant to become entombed and 
preserved in the fossil state is that the locality 
in which it grows shall become somewhat rap- 
idly covered up, burying the entire plant so 
quickly that it cannot decay during the process. 
This, as anyone can see, must be an exceedingly 
rare occurrence. Still, there is no doubt that a 
large amount of monocotyledonous vegetation 
growing in bogs and marshes in estuarine regions 
that are slowly subsiding under the weight of 
materials brought down the streams, and which 
also aid in covering them up, has been, in fact, 
preserved in a very imperfect way, and many 
vague and puzzling objects occur in all collec- 
tions made from such localities. They are 
SCIENCE. 
103 
found throughout the Mesozoic, in the form of 
short culm-like segments and imperfect bits of 
leaves so badly macerated that they are ne- 
glected by those who determine such collections. 
It is rarely possible to say what form of plant 
they really represent, and yet itis often clear 
that these remains belong to certain glumaceous 
forms, grasses, sedges, rushes, etc. Saporta, 
in his work on the Mesozoic of Portugal, de- 
scribed and figured, under the name of Poac- 
tites, quite a number of these forms from the 
Neo-Jurassic to the Albian, or through the Upper 
Jurassic and entire Lower Cretaceous. Others 
have been called Cyperites, Zosterites, Bambu- 
sium, etc. Numerous small seeds are also con- 
stantly occurring, which are for the most part 
unnamed or given such names as do not indi- 
cate their botanical affinities. Many of these 
probably belong to monocotyledonous plants. 
Mr. Tyler’s paper, with all its excellencies, 
conveys the impression of an unfinished pro- 
duction. One would say that in his hurry to 
use it as a thesis he had been obliged to close it 
up abruptly. Its most serious defect is the want 
of careful descriptions of the plates and figures 
explained in their numerical order for the con- 
venient use of the reader. This condition of 
the paper suggests the probability that the 
writer has much additional material, and in- 
spires the hope that he may have entered upon 
a much more extended and exhaustive series of 
observations along these suggestive lines. 
LESTER F’. WARD. 
On the Genera of Rodents: An Attempt to bring 
up to Date the current Arrangement of the 
Order. By OLDFIELD THOMAS, F.Z.S. Proc. 
Zool. Soc., London, 1896, pp. 1012-1028. 
Issued April, 1897. 
The order Rodentia offers peculiar difficulties 
to the student, both on account of the number 
of its species and the great variety of forms 
which it includes. The satisfactory arrange- 
ment of the thousand or more species now 
known is no easy matter, as shown by the at- 
tempts of several authors, notably Waterhouse 
in 1839-48, Gervais in 1848-53, Brandt in 1855, 
Lilljeborg in 1866, Gill in 1872, and Alston in 
1876. During the last ten years more progress 
has been made in the study of mammals than 
