The complete structure and zoological relationship of the trilobites has always been open to some doubt. As regards the former, within recent years exceptionally well-preserved specimens from the Utica Slates and the Cincinnati Limestone of Ohio, rocks of Ordovician age, have been discovered and dissected, whereby our knowledge of the organisation of this group is greatly advanced. These remarkable fossil remains show that the Trilobites bore on their under surface a number of appendages, one pair to each segment, except that of the anal. The front pair is whip-like and served as antennae; the others are branched, the forward portion being a crawling limb, and the hinder, which was fringed with bristles or thin plates, may have served either for swimming or breathing. At the base of the four pairs of appendages attached to the head there was an arrangement for biting the food, from whence it was passed to the mouth. Taking one of the commonest Australasian trilobites, _Dalmanites meridianus_, for an example of general structure, and looking at the back of the shell or upper surface, we see the trilobate (three-lobed) form well defined (Fig. 107). The central ridge is termed the axis, and on either side of this are arranged the pleural lobes, each well marked transverse division of which, in the central or thoracic region, being a pleuron or rib. The whole body is divided into three more or less distinct portions,--the head-shield or cephalon, the thorax, and the tail-shield or pygidium. The central area of the head-shield is called the glabella or cranidium, against which, on either side, are placed the free cheeks carrying the compound sessile eyes when present. The appendages of the head are pediform or leglike, arranged in five pairs, and biramous or forked, excepting the antennae, which are simple and used as sensory organs. In front of the mouth is the hypostoma or forelip, and behind it is the metastoma or hind-lip. The segments of the head-shield are most closely united, and in all the trilobites are of the same number. Those of the thorax have flexible joints and are variable in number. The segments of the abdomen are fused together and form a caudal shield or pygidium. The larval stage of the trilobite was a protonauplian form (that is more primitive than the nauplius), the protoaspis; the adult stage, being attained by the addition of segments at the successive moults. The earliest known trilobites in Australia are some Cambrian species from South Australia, Western Australia, Victoria, and Tasmania. =Lower Cambrian Trilobites.--= [Illustration: =Fig. 108--CAMBRIAN TRILOBITES.= A--Ptychoparia howchini, Eth. fil. L. Cambrian. South Australia B--Dolichometopus tatei, H. Woodw. L. Cambrian. South Australia C--Agnostus australiensis, Chapm. Up. Cambrian. Victoria D--Ptychoparia thielei, Chapm. Up. Cambrian. Victoria E--Dikellocephalus florentinensis, Eth. fil. L. Cambrian. Tasmania ]