OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 229 
sive, and a few are said to be common to the Carboniferous and 
the Permian rocks. The most conspicuous of these are Pleu- 
votomaria (H.) carinata, Sow., Spirifer (T.) cristata, Schloth., 
Terebratula elongata, Schloth. and Fenestella (Gorg.) antiqua, 
Goldf. After carefully examining the specimens from the Mag- 
nesian-limestone, and comparing them with the descriptions and 
figures of those with which they have been united, it appears from 
characters that will be stated under each species, that Plew- 
rotomaria carinata does not occur in the Magnesian-limestone, 
that Spirifer cristata is quite distinct from Spirifer octopli- 
cata, Sow., that the Terebratula elongata is very different from 
T. hastata, Sow., and that the Penestella (G.) antiqua, Goldf. 
is not identical with the species referred to it by Mr. M‘Ooy, 
Prof. Phillips, and Prof. Lonsdale. This reference, however, by 
such distinguished authors of species peculiar to one system to 
those peculiar to another, though not satisfactory as it regards 
species, is yet the strongest proof that can be given of the 
close relations of the faunas of these two systems. 
Many of the genera which occurred and were common in the 
Carboniferous rocks did not ascend into the Permian system. 
Of these the most remarkable are the genera Orthoceras, Belle- 
rophon and Goniatites among the Cephalopoda ; Kuomphalus, 
Cirrus and Loxonema among the G'asteropoda ; Posidonia and 
Pleurorhynchus among the Conchifera ; Leptena and Chonetes 
among the Brachiopoda ; the whole family of Trilobites among 
the Crustacea ; and many families and genera among the Hchino- 
dermata and Zoophyta. This loss of families and genera is at- 
tended with a diminution in the number of species, and in the 
size of the individuals of those genera which have survived. 
Though the Brachiopoda are sufficiently numerous in the Per- 
mian system, andare still large enough to be considered the most 
characteristic group, yet some of them occur in these rocks for 
the last time, as the genus Orthis; and several genera, as Pro- 
ductus and Spirifer, which are found again in more recent 
systems, are so dwindled in size and are so few in number as to be 
easily overlooked. For the sake of comparison and in proof of 
the great diminution of species, the number of fossils which occur 
