214 
SILURIA. 
[Chap. IX. 
Acidaspis Brightii, Murch., and Cheirurus bimucronatus, Murch., with the small 
Cyphaspis megalops, M'Coy, and Proetus latifrons, M'Coy. The most common 
of all is Encrinurus punctatus, Briinn., whilst Calymene Blumenbachii, Brongn., 
which ranges from the Snowdon or Caradoc rocks to the Upper Ludlow, is not 
unfrequent. Lastly, it is also to be observed that a fragment of Pterygotus was 
detected by the late H. E. Strickland in the Malvern district. 
No traces of Fish-remains have been discovered in British deposits of this age, 
nor in the overlying Wenlock rocks. 
The reader who has perused the preceding details respecting the distri- 
bution of organic remains, as well as those former chapters which treat of 
the ascending series of Silurian deposits constituting the system, will have 
perceived that the sharp distinction which was at first supposed to exist 
between the Lower and Upper Silurian rocks no longer holds good. When 
my classification was proposed in the year 1835, scarcely one of those 
species which were considered typical of the inferior division had been ob- 
served to pass upwards into the superior group ; and yet the community of 
genera convinced me that the whole should be united in one natural 
system. Seeing that the type shell of the Llandovery and May Hill Sand- 
stones, Pentamerus oblongus, Sow., was never detected in the overlying 
"Wenlock Shale, and was associated with certain species which belonged 
to the lower part of the series, — and influenced also, to some extent, by 
mineral characters, I classed the former as the summit of my Lower Si- 
lurian rocks. 
With the enlargement, however, of the field of examination, not only by 
the Survey of all Wales, but by researches in Ireland and Scotland, and by 
the more exact comparison of the fossils, it has become evident that, whilst 
a great number of species pervade nearly the whole system, the zone so 
laden with Pentameri, to which the term Llandovery Rocks has been ap- 
plied, is truly of intermediate character. In short, its inferior member 
contains many Lower Silurian types, and its superior strata are unques- 
tionably more connected with the Upper Silurian group (of which, through- 
out many tracts, they form the natural physical base), and are locally 
transgressive to the inferior rocks. Hence, in the legend attached to the 
Map, the Lower Llandovery is grouped with the inferior division, the 
Upper Llandovery, though still characterized by the same Pentameri as the 
lower mass, being linked on to the superior strata. 
It is, indeed, important to give a prominence to this connecting zone by 
treating separately of its characteristic fossils ; for, as will hereafter ap- 
pear, there are foreign countries (Scandinavia and Russia) in which there 
is no discordance of strata, and where the stratigraphical and lithological 
passage from Lower to Upper Silurian is in perfect harmony with the 
zoological transition above described. 
