I08 HISTORICAL PALAEONTOLOGY. 



richia (fig. 47, e), and Leperditia (fig. 47, / and j). 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, /), 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 Caryocarts (fig. 47, d) and 

 Ceratiocaris. ^lore important, however, than any of these are 

 the Trilobites. which may be considered as attaining their maxi- 

 mum development in the Lower Silurian. The huge Faradoxides 

 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 X\\^ gY^.2X Asaphus tyraimus 

 of the Upper Llandeilo (fig. 47, a), 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 Trilo- 

 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, Og}'gia, Cheirurus, Avipyx^ Calyjuene, Trinudeus^ 

 Lichas, Illcemis, ^^glhia, Harpes, Reiiiopleiirides, Phacops, 

 Acidaspis, and Homalonotus, a few of them passing upwards 

 under new forms into the Upper Silurian. 



Coming next to the MoUusca^ we find the group of the Sea- 

 mosses and Sea-mats {Polyzod) represented now by quite a 

 number of forms. Amongst these are examples of the true 

 Lace-corals {Retep07'a and Fenestelld), 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 {Hippothoa, Alecto, &c.); but perhaps the 

 most characteristic forms belong to the genus Ptilodictya (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 B?-achiopods are so numerous, and pre- 

 sent such varied types, both in this and the succeeding period 

 of the L^pper Silurian, that the name of '•' Age of Brachiopods" 



