_LINKS BETWEEN THE INVERTEBRATES AND FISHES._--The curious little ascidians or "sea-squirts," belonging to the group Tunicata, are held by some authorities to be the degenerate descendants of a free-swimming animal having a complete notochord and nerve-tube, structures which are now only seen in the tails of their tadpole-like larvae. The fully developed tunicate is generally sessile and provided with a thick outer coat (tunic) and muscular inner lining. This outer coat in some forms, as _Leptoclinum_, is strengthened with tiny calcareous spicules, and these are sometimes found in the fossil state in Cainozoic clays, as well as in some of the calcareous deep-sea oozes. The little stellate spicules of _Leptoclinum_ are abundant in the Balcombian clays of Mornington, Victoria. Another primitive form with a notochord is the Lancelet, but this, having no hard parts, is not found in the fossil state. =Primitive Types of Fishes.--= _FISHES._--The remains of fishes are naturally more abundant in the fossil condition, owing to their aquatic habits, than those of other vertebrates. The earliest fishes were probably entirely cartilaginous, and some have left only a mere trace or impression on the rocks in which they were embedded. These primitive fishes have no lower jaw, and are without paired limbs. They are sometimes placed in a class by themselves (_AGNATHA_). The orders of this primitive fish series as represented in Australasia are the Osteostraci ("bony shells"), of which the remains of the _Cephalaspis_-like head-shield of _Thyestes_ has been found in the Silurian of N.E. Gippsland, Victoria (Fig. 122); and the Antiarchi, with its many-plated cuirass, armoured body-appendages, internal bony tissue, and coarsely tuberculated exterior, as seen in _Asterolepis australis_, a fossil occasionally found in the Middle Devonian Limestone of Buchan, Gippsland. =True Fishes.--Devonian.--= Of the true fishes (Pisces), the Elasmobranchii ("slit-gills"), a sub-class to which the modern sharks belong, are represented in the Devonian series by the paired spines of a form resembling _Climatius_, found both in Victoria and New South Wales. Remains of Dipnoi ("double-breather" or lung-fishes) occur in the Devonian of Barker Gorge, Western Australia, represented by a new species allied to _Coccosteus_ ("berry-bone" fish); and in a bed of the same age at the Murrumbidgee River, New South Wales by the cranial buckler of _Ganorhynchus süssmilchi_. [Illustration: =Fig. 122--Incomplete Head-Shield= of Thyestes magnificus, Chapm. From the Silurian (Yeringian) of Wombat Creek, N.E. Gippsland. 4/5 nat. size] [Illustration: =Fig. 123= =Gyracanthides murrayi=, A. S. Woodw. L. Carboniferous. Mansfield, Victoria. (Restoration). About 1/12 nat. size] [Illustration: =Fig. 124--TEETH and SCALES of PALAEOZOIC and MESOZOIC FISHES.= A--Strepsodus decipiens, A. S. Woodw. L. Carboniferous. Victoria B--Elonichthys sweeti, A. S. Woodw. L. Carboniferous. Victoria