Ostracoda swarmed in many of the streams, lakes and seas of past geological times, and they still exist in vast numbers under similar conditions. Like some other minute forms of life, they played a most important part in building up the rock formations of the sedimentary series of the earth's crust; and by the decomposition of the organism itself they are of real economic value, seeing that in some cases their decay resulted in the subsequent production of oil or kerosene shales and bituminous limestones. The Carboniferous oil shales in the Lothians of Scotland, for example, are crowded with the carapaces of Ostracoda associated with the remains of fishes. =Cambrian Ostracoda.--= Some undescribed forms of the genus _Leperditia_ occur in the hard, sub-crystalline Cambrian Limestone of Curramulka, South Australia. =Silurian Ostracoda.--= In Victoria and New South Wales the oldest rocks from which we have obtained the remains of Ostracoda up to the present, are the uppermost Silurians, in which series they occur both in the limestone and the mudstone. In Victoria their bivalved carapaces are more often found in the limestone; but one genus, _Beyrichia_, is also met with in abundance in the mudstone. These mudstones, by the way, must have originally contained a large percentage of carbonate of lime, since the casts of the shells of mollusca are often excessively abundant in the rock, and the mudstone is cavernous, resembling an impure, decalcified limestone. These Yeringian mudstones of Victoria seem, therefore, to be the equivalent of the calcareous shales met with in the Wenlock and Gotland Series in Europe; a view entirely in accordance with the character of the remainder of the fauna. One of the commonest of the Silurian ostracods is _Beyrichia kloedeni_, a form having an extensive distribution in Europe. It occurs in the Silurian mudstone of the Upper Yarra District. Other species of the same genus are _B. wooriyallockensis_ (Fig. 112 A), distinguished from the former by differences in the shape of the lobes and its longer valves; also a form with narrow lobes, _B. kilmoriensis_; and the ornate _B. maccoyiana_, var. _australis_. Of the smooth-valved forms, mention may be made of _Bythocypris hollii_, _B. caudalis_ (Fig. 112 D), and the striking form, _Macrocypris flexuosa_. Regarding the group of the _Primitiae_, of which as many as thirteen species and varieties have been described from the Lilydale Limestone, we may mention as common forms _P. reticristata_ (Fig. 112 E) and _P. punctata_. This genus is distinguished by the bean-shaped or purse-shaped carapace, with its well developed marginal flange and mid-dorsal pit. Other genera which occur in our Silurians and are of great interest on account of their distribution elsewhere. are _Isochilina_, _Aparchites_, _Xestoleberis_, _Aechmina_, and _Argilloecia_. [Illustration: =Fig. 112--SILURIAN OSTRACODA.= A--Beyrichia wooriyallockensis, Chapm. Silurian (Yer.) Victoria