Trilobites are well-known fossils in the Australasian Silurian strata. As they occur rather abundantly along with other fossils in rocks of this age they are extremely useful aids in separating the system into the different beds or zones. In Victoria the Silurian is divisible into two sets of beds: an older, or Melbournian stage (the bed-rock of Melbourne) and a younger, Yeringian (Lilydale series). Trilobites of Melbournian age are found to belong to the genera _Ampyx_, _Illaenus_, _Proetus_, _Cyphaspis_, _Encrinurus (Cromus)_ and _Homalonotus_. The commonest species are _Cyphaspis spryi_ (Fig. 109 B), and _Encrinurus (Cromus) spryi_ from the South Yarra mudstones; and _Ampyx parvulus_, var. _jikaensis_ (Fig. 109 A), and _Homalonotus harrisoni_ (Fig. 109 C), from the sandstone of Moonee Ponds Creek. The handsome _Dalmanites meridianus_ and _Homalonotus vomer_ occur at Wandong in what appear to be passage beds between the Melbournian and Yeringian. The Yeringian of Victoria is far richer in trilobites than the preceding series, and includes the genera _Proetus_, _Cyphaspis_, _Bronteus_, _Lichas_, _Odontopleura_, _Encrinurus_, _Calymene_, _Homalonotus_, _Cheirurus_, and _Phacops_. The rocks in this division occur as mudstones, limestones, and occasionally sandstones and conglomerates. The mudstones, however, prevail, and these pass insensibly into impure limestones of a blue-black colour, weathering to brown, as at Seville; the change of structure indicating less turbid water. At Lilydale, and on the Thomson River, as well as at Loyola and Waratah Bay, almost pure limestone occurs, which represents clear water conditions, not necessarily deep; there, however, trilobites are scarce, and the prevailing fauna is that of an ancient coral reef. Some described Yeringian species are _Lichas australis_ (Fig. 110 A), _Odontopleura jenkinsi_ (Fig. 110 B) (found also in New South Wales), _Encrinurus punctatus_ (Fig. 110 C), _Calymene tuberculosa_, _Bronteus enormis_, _Phacops sweeti_, and _P. serratus_ (Fig. 110 E). In _Calymene_ ("covered up") the joints of the thorax are facetted at the angles, so that each pleuron could work over that immediately behind; in consequence of this it could roll itself up like a woodlouse or slater, hence the name of the genus. This trilobite also occurs in England, and is there known amongst the quarry men and fossil collectors as the "Dudley Locust." Perhaps the most characteristic and common trilobite of the Yeringian series in Victoria is _Phacops sweeti_ (Fig. 110 D), formerly identified with Barrande's _P. fecundus_, from which it differs in the longer and larger eye with more numerous lenses. It is found in Victoria in the Upper Yarra district near the junction of the Woori Yallock and the Yarra Rivers; north-west of Lilydale; near Seville; at Loyola near Mansfield; and at Fraser's Creek near Springfield, Kilmore. [Illustration: =Fig. 110--NEWER SILURIAN TRILOBITES.=