268 
T. Davidson — What is a Brachiopod ? 
comparative anatomy, so essential to tke proper understanding of 
fossil, or extinct genera and species. 
But to return to our subject. I cannot do better tban to extract 
front Lyell tbe following passage. “ Notbing is more remarkable, 
in tbe Silurian strata generally of all countries, than tbe pre- 
ponderance of tbe Bracbiopoda over otber forms of Mollusca. Their 
proportional numbers can by no ineans be explained by supposing 
tkem to have inkabited seas of great depth, for tbe contrast between 
tbe palseozoic and tbe present state of tbings bas not been essentially 
altered by tbe late discoveries made in our deep-sea dredgings. We 
find tbe living Bracbiopoda so rare as to form about one forty-fourth 
of tbe wliole bivalve fauna, wbereas in tbe Lower Silurian rocks, and 
where the Bracbiopoda reacli their maximum, tbey are represented 
by more tban twice as many species as tbe Lamellihranckiate bi- 
valves. Tkere may, indeed, be said to be a continuous decrease of 
tbe propoi-tional number of this lower tribe of Mollusca as we pro- 
ceed from the older to tbe newer rocks.” 
Dr. Bigsby informs me tbat from a conspectus in bis new The- 
saurus (now in tbe press) tbe following numbers of Silurian, Devo- 
nian, and Carboniferous Brachiopoda have been gi ven, with due re- 
gard to accuracy, but tbat after all it is ouly a careful approximation 
liable to future disturbances : 
America. 
Europe, etc. 
Total. 
Cambrian and Silurian 
689 
733 
1422 
Devonian 
577 
789 
1366 
Carboniferous 
488 
384 
871 
1754 
1905 
3659 
The number from tbe Pfermian formation, wliicb completes the 
Palseozoic series, has not yet been computed, but they are compara- 
tively few in number. Making a large allowance for synonyms, it 
will be seen tbat fully 3000 species are already known to have 
existed during tbe primary periods. It is also a remarkable fact tbat 
the Bracbiopoda, so immensely abundant during tbe Cambrian, 
Silurian, Devonian, and Carboniferous periods, became apparently 
very muck less numerous during the Permian and Triassic ; while 
tbey again became abundant, althougb comparatively reduced in num- 
ber, during the Jurassic and Cretaceous period. In tbe Tertiaries 
tbey bad material ly decreased in number, and are represented, at the 
present time, by about one bundred species. It bas also been clearly 
ascertained tbat a certain number of genera and species passed from 
one System or formation into tbe one that followed it, as may be 
seen by a glance at the Table, in whicb tbe general distribution of 
tbe genera in time has been given witb as much accuracy as the 
present state of our knowledge will admit. From this table it will 
be perceived that about 9 genera appeared for the first time in the 
Cambrian sj'stem, 52 in tbe Silurian, 21 in tbe Devonian, 7 Carbon- 
