Chap. III.] LOWER SILURIAN GKAPTOLITES. 
Gl 
clearly recognizable through their position and organic remains, it is not 
pretended that a line can everywhere be precisely drawn, upon a map, 
between them and the Caradoc formation; and thus on the accompanying 
Map the tint 2 b is shaded off into the colour of 2 C . In all such cases of 
junction and passage, the general term ' Lower Silurian ' can alone be with 
safety applied. 
Graptolites. — In terminating this sketch of the Llandeilo formation, a few 
words may be offered on the fossils called Graptolites, already partially 
adverted to, and which, wherever they are found, clearly mark the rock 
to be Silurian. They occur, indeed, in North and South America, Britain, 
Scandinavia, Germany, Eussia, Prance, and Spain. Abounding in the 
lowest strata of the Llandeilo formation, which graduate downwards into 
the Lingula-flags, these Graptolites are specially abundant in the very tract 
of Shropshire (Stiper Stones to Shelve) on which such stress has been laid as 
affording the best illustration of a large mass of the inferior strata. They 
prevail, in fact, throughout the Silurian rocks wherever the nature of the 
sediment has been favourable to their existence, — that is, in schistose argil- 
Fossils (12). Lower Silurian Graptolites. 
1. Rastritesf peregrinus, Barrande. 2. Graptolithus Sedgwickii, Portlock. 3. G. 
priodon, Bronn (G. Ludensis, Sil. Syst.). 4. Climacograptus scalaris, Linn. 4*. Di- 
plograpsus folium, Hisinger. 5. D. nodosus, Harkness (a doubtful species). 6. D. fo- 
lium, Hisinger (young specimen). 7. Dicranograptus ramosus, Hall. 8. Didymo- 
grapsus sextans, Hall. 9. D. Murcbisonii, Beck. 
laceous strata, or the finely levigated muddy bottoms of the primeval seas. 
They are supposed by many naturalists to have been Zoophytes nearly 
allied to the living Yirgularia, a creature known only in deep water. 
Others rather consider these extinct forms to belong to Sertularian Zoo- 
phytes, or even to Polyzoa£. Be this as it may, the geologist has observed 
that they are found exclusively in the Silurian system of life. The whole 
t See G-einitz (Verstein. G-rauw. -Format.) who, much more complex in structure than any here 
for the sake of uniformity, groups all the G-rap- figured. 
tolites with a single row of teeth (Graptolithus, I Mr. Salter argues for this latter view of their 
Eastrites, &c.) under the name Monograpsus. affinities in the Appendix to the 'Geology of North 
Some forms have been discovered in Canada Wales,' Memoirs Geol. Survey, vol. iii. 1866. 
