The limestone beds at Laurie’s Creek and other localities in Central Australia contain remains of Asaphus illarensis, A. howchini and A. lissopelta; whilst in the limestone and quartzite of Middle Valley, Tempe Downs, A. thorntoni also occurs.
Fig. 109—OLDER SILURIAN TRILOBITES.
A—Ampyx parvulus, Forbes, var. jikaensis, Chapm. Silurian (Melb.) Victoria
B—Cypaspis spryi, Gregory. Silurian (Melb.) Victoria
C—Homalonotus harrisoni, McCoy. Silurian (Melb.) Victoria
D—Phacops latigenalis, Eth. fil. and Mitch. Silurian. N.S. Wales
Silurian Trilobites.—
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.