suESS — nisrnir.uTro.v ok tuk r.nAcnnopoDA. 
287 
I i)i'c;-inmc that the bathymetrical distribution of the gi'eat groups 
ol' Unvchiopotla has ever been nearly the same as it is now, viz., the 
species with a translucid slieli being confined, or nearly so, to the 
higher zones, and tliose with opaque shells living in the deeper parts 
of tlie sea. It is not necessary to enumerate the numerous cases in 
which fossil Lingulidiia and Discinidos have been found, together with 
ripple-marks, littoral shells, or other signs of shallow sea ; nor to 
point out how rare it is to meet with other Brachiopoda in such con- 
ditions. But I must beg you to remember how very predominant 
hoi'idike shells of Brachiopoda are in sandstone and shale, and how 
mai'lced the maxima of Rhynchonellida3, Spiriferidre, Terebratulidae, 
&c., are in limestone, and more especially in argillaceous limestone. 
It is a gi'oss error to believe that these families are so frequently 
wanting in sandstones or shales, because these latter have been 
formed by a sea too poor in lime for the formation of these shells. 
MM. Logan and Hiant have taiight us that the shells of LinguliB also 
consist principally of lime. I believe that the extinct family Spiri- 
ferida3 ranges, as I have just remai-ked, with the other families with 
opaque shells ; but as to the Strophomenidte you will see that excep- 
tions are somewhat more numerous, most of them belonging to the 
(restricted) genus Oiihis. And now let us cast a glance on the dis- 
tribution of a number of Silurian Brachiopoda, and for the sake of 
shortness let us call all Lingulidee and Discinid^ the Grouj} A, and 
all other Brachiopoda the Oroup B, the genus Orthis and a number 
of other Strophomenidas being regarded as standing between these 
groups, Orthis, as you will remark, often ofiering a very marked 
tendency associating with gi'oup A. 
In looking, as a first example, at M. Barrande's newest list of 
primordial fossils in the Bulletin Soc. Geol. xvi., 1859, p 516 — 546, 
you may find the following Brachiopoda : — Eight species of Lingula, 
two Obelus, three Discina, five Orthis, and one doubtful Atryjja. 
This can only be Atr. nucula (Dalne), or Atr. lenticularis (Dalne),the 
first of which, according to M. BaiTande, approaches to Obolus or Lin- 
gula. The latter surely is one of the Sti'ophomenidfe, and probably an 
Orthis, So you see that this fauna only offers thirteen or fourteen 
members of the group A, and five or six forms of the genus Orthis. 
I camiot believe that this association of Brachiopoda has lived in deep 
water, and you know that the predominant rocks in which they are 
found are sandstones and slates. 
In this case it is possible to attain a marked conclusion even in 
contemplating the primordial fauna of all regions in which it has 
been found as a whole, but for further studies it is necessary to 
regard one country after the other. I will select a few examples 
now, but must, before I do so, say that a part of the conclusions 
at which I have arrived has been anticipated in a very nice way 
by the statement of Mr. Sharpe (Quart. Joui-, 1848, iv., p. 158), 
that the genera Ling-ula and Orbicula, now preferring shallow 
water, do not offer identical species in the Silurian beds of 
North America and Euro^ie. I beg you to read the passage of 
