Reviews—Mineral Resources of Great Britain. 523 
If it were verified, this character would be much more important 
than a mere coincidence in the number of segments. 
On one point the student of recent Crustacea wil] certainly dissent 
from Professor Raymond’s conclusion. As. he states, “Since the 
earliest times there has been a constant temptation to compare the 
depressed shields of the trilobites with the similar ones of !sopods,”’ 
and he has succumbed to this temptation so far as to suppose that 
the Isopods may have been derived directly from Trilobites and 
independently of the other Malacostraca. It can be stated with 
some confidence that this view receives no support from what we 
know of the structure of recent Isopods. If it is ever permissible 
to draw phylogenetic conclusions from the facts of comparative 
anatomy, then it is as certain as anything of the sort can well be that 
the Isopoda have been evolved from a primitive Mysidacean type. 
The intermediate steps of this descent are indicated not obscurely, 
although overlain by divergent specializations, in the Cumacea and 
Tanaidacea. The resemblances in general aspect between certain 
Trilobites and certain Isopods are very remarkable indeed, but the 
view that they indicate descent of the one group from the other 
can only he compared with those fantastic speculations which 
would derive whales from Ichthyosaurs and bats from Pterodactyls. 
Professor Raymond’s discussions of the course of evolution within 
the group of Trilobites, and of their probable habits and mode of 
life, are especially interesting and suggestive. The whole memoir, 
however, deserves careful study by all students of fossil and recent 
Arthropoda. An index would have helped in its use, but this want 
is to some extent supplied by a full and methodically arranged table 
of contents. A word of praise is due to the excellent portrait of 
Professor Beecher which forms the frontispiece. 
W. T. Carman. 
SPECIAL REPORTS ON THE MINERAL RESOURCES OF GREAT BrivAty. 
Memoirs of the Geological Survey. 
Vou. XIX: Leap AND Zinc ORES IN THE CARBONIFEROUS Rocks 
oF NortH Wates. By Brernarp Smit, M.A. pp. iv + 162, 
with 3 plates and 25 figures. 1921. Price 5s. 6d. net. 
Vout. XXI: Leap, SILVER-LEAD, AND Zinc ORES OF CORNWALL, 
Devon, AND Somerset. By Henry Dewey. pp. iv + 72, 
with 4 plates and 14 figures. 1921. Price 2s. 6d. net. 
Vou. XXII: Tue Leap anp Zrtnc Ores oF THE LAKE DISTRICT. 
By T. Eastwoop. pp. iv+ 56, with | plate and 4 figures. 
1921... Price 2s; net: 
(CYNE of the most difficult and interesting problems still confronting 
the scientific student of metallogeny is the elucidation of the 
origin of the ores of lead and zinc that are found in such large 
quantities in limestone strata of various ages. At one time it was 
generally believed that the vein-fillings had been deposited from 
meteoric waters percolating downwards, or by some form of “ lateral 
