216 
SILURIA. 
[Chap. X. 
system, allusion has already been made (Chap. IX.), and notice will now 
be taken of other forms, of like duration in time, which are more abundant 
in the upper division. The attention, however, of the reader will be directed 
chiefly to the types which are peculiarly, and, as far as our present know- 
ledge goes, exclusively, Upper Silurian. (See Plates XIII.-XXXY.) 
The Graptolites, which, as before stated, are so very abundant in the shaly 
and schistose portions of the Lower Silurian of Wales and Scotland, become 
much more scarce as to species in the Upper Silurian ; and though there are 
many tracts where the Wenlock Shale and Lower Ludlow rocks are crowded 
with them, it is only one species * (Graptolithus priodon) that is abundant in 
these upper divisions of the system. When argillaceous sediments abound in 
these rocks, the Graptolite is rarely absent; but with the cessation of such 
peculiar conditions this zoophyte, which must have grown on the fine mud at 
the bottom of the sea, disappears. 
One of these Graptolites, which occurs in the old rocks of Snowdon, ranges 
through the series even to the Upper Ludlow rocks, and hence was named 
G. Ludensis before it was identified with G. priodon. In the Wenlock Shale, 
especially towards the base of that deposit, this fossil is sometimes accompanied 
by a very beautiful and peculiar species — the Retiolites Geinitzianus of Barrande. 
The latter occupies the same position (at the base of the Upper Silurian series) 
in Bohemia, whence it was first described. It is a Double Graptolite, like the 
genus Diplograpsus (see PI. I. f. 2), from which, however, it differs essentially 
in having no central axis, and in having the surface composed of a reticular 
tissue instead of a continuous horny coat. An excellent figure of it is given in 
M. Barrande 's memoir 'On the Graptolites of Bohemia.' Above the Ludlow 
rocks, and throughout the whole series of overlying Palaeozoic strata, no true 
Graptolite, as before observed, has ever been found ; and I repeat, therefore, that 
this zoophyte is a marked Silurian type. 
1. Polypora? crassa (Hornera, Sil. Syst). 2. Fenestella assimilis (Gorgonia, Sil. 
Syst.) ; a common Wenlock species: 3. F. Lonsdalei, D'Orb. (F. prisca, Sil. Syst.). 
4. F. Milleri, Lonsd. 5. Glauconome disticha, Goldf. 6. Ptilodictya lanceolata, Goldf., 
a reduced figure of a full-grown specimen ; a part is figured above it, natural size. 
7. Pt. lanceolata, in the young state ; the cells are longer in proportion. 
It should further be observed that in Britain neither the Double or foliaceous 
Graptolite (Diplograpsus) nor the Twin Graptolite (Didymograpsus) is ever meU 
with above the horizon of the Caradoc formation. They are therefore excellent 
* Another species has been detected (but only Flemingii, Salter, from Kirkcudbright Bay, where 
in one locality, where it is plentiful) in strata be- it was first collected by the Earl of Selkirk. See 
lieved to be of Wenlock age. It is Graptolithus Quart. Journ. Greol. Soc. vol. viii. pi. 21. f. 5, 6. 
Fossils (50). Upper Silurian Polyzoa. 
