108 HISTORICAL PALAZONTOLOGY. 
richia (fig. 47, e), and Leperditia (fig. 47, ¢ and 7). Most of 
these are very small, varying from the size of a pin’s head up 
to that of a hemp seed; but they are sometimes as large as 
a small bean (fig. 47, z), and they are commonly found in 
myriads together in the rock. As before said, they belong to 
the same great group as the living Water-fleas (Ostracoda). 
Besides these, we find the pod-shaped head-shields of the 
shrimp-like Phyllopods—such as Caryocaris (fig. 47, @) and 
Ceratiocaris. More important, however, than any of these are 
the Z7ilobites, which may be considered as attaining their maxi- 
mum development in the Lower Silurian. The huge Paradoxides 
of the Cambrian have now disappeared, and with them almost 
all the principal and characteristic ‘‘ primordial” genera, save 
Olenus and Agnostus. In their place we have a great number 
of new forms—some of them, like the great Asaphus tyrannus 
of the Upper Llandeilo (fig. 47, @), attaining a length of a foot 
or more, and thus hardly yielding in the matter of size to their 
ancient rivals. Almost every subdivision of the Lower Silurian 
series has its own special and characteristic species of Tnlo- 
bites ; and the study of these is therefore of great importance 
to the geologist. A few widely-dispersed and characteristic 
species have been here figured (fig. 47); and the following 
may be considered as the principal Lower Silurian genera— 
Asaphus, Ogygia, Chetrurus, Ampyx, Calymene, Trinucleus, 
Lichas, LMenus, Aglina, Harpes, Remopleurides, Phacops, 
Acidaspis, and Homalonotus, a few of them passing upwards 
under new forms into the Upper Silurian. 
Coming next to the AZo//usca, we find the group of the Sea- 
mosses and Sea-mats (Po/yzoa) represented now by quite a 
number of forms. Amongst these are examples of the true 
_ Lace-corals (Retepora and Fenestella), with their netted fan-like 
or funnel-shaped fronds; and along with these are numerous 
delicate encrusting forms, which grew parasitically attached to 
shells and corals (Hipfothoa, Alecto, &c.); but perhaps the 
most characteristic forms belong to the genus P¢/odictya (figs. 
48 and 49). In this group the frond is flattened, with thin 
striated edges, sometimes sword-like or scimitar-shaped, but 
often more or less branched ; and it consists of two layers of 
cells, separated by a delicate membrane, and opening upon 
opposite sides. Each of these little chambers or “ cells” was 
originally tenanted by a minute animal, and the whole thus 
constituted a compound organism or colony. 
The Lamp-shells or 4rachiopods are so numerous, and pre- 
sent such varied types, both in this and the succeeding period 
of the Upper Silurian, that the name of “ Age of Brachiopods” 
