474 
ELEMENTS OF GEOLOGY. 
Great Britain eleven genera and about 40 species of Grap- 
tolites occur in the Llandeilo flags and underlying Arenig 
beds. The double Graptolites, or those with two rows of 
cells, such as Diplograpsus (Fig. 557), are conspicuous. 
The brachiopoda of the Llandeilo flags, which 
number 47 species, are in the main the same as ' 
those of the Caradoc Sandstone, but the other 
mollusca are in great part of diflerent species. 
In Europe generally, as, for example, in Sweden 
and Russia, no shells are so characteristic of this 
Diplograpsus fo- formation as Orthoceratites, usually of great size, 
Dumfdesshfi^e • ^nd with a wide siphuncle placed on one side in- 
Swedeii. Lian- gtead of being central (see Fig. 560). Among 
ei o ags. Cephalopods in the Llandeilo flags is Cyr- 
toceras; in the same beds also are found Bellerophon (see Fig. 
488, p. 436) and some Pteropod shells ( Conularia^ Theca^ Gtc.), 
Fig. 560. 
Orthoceras duplex, Wahlenberg. Russia and Sweden. 
(From Murchison’s “Siluria.”) 
a. Lateral siphuncle laid bare by the removal of a portion of the chambered shell. 
h. Continuation of the same seen in a transverse section of the shell. 
also in spots where sand abounded, lamellibranchiate bivalves 
of large size. The Crustaceans were plentifully represented 
by the Trilobites, which appear to have swarmed in the Si- 
Fig. 559. 
Fig. 561. 
Asaphus tyrannus, Murchison. 
Llandeifo; Bishop’s Castle; 
etc. 
Fig. 562. 
Ogygia BucMi, Burm. Syn. Asaphus Buchii, 
Brongn. Builth, Radnorshire; Llandeilo, 
Carmarthenshire. 
lurian seas just as crabs and shrimps do in our own ; no less 
than 263 species have been found in the British Silurian 
fauna. The genera Asaphus (Fig. 561), Ogygia (Fig. 562), 
