OSTRACODS FROM SOME QUEENSLAND TERTIARY BASINS. 103 
Range : No Tertiary representative of the recent genus Ilyodromus 
appears to have been recorded previously. 
Ilyodromus ? concentricus, sp. nov. Plate VI, figs. 2 a, b. 
Holotype: F. 1977, Queensland Geological Survey Collection. 
Locality: Bore 1, at 157 feet. Parish of Bundle,, County of Deas 
Thompson, between Gladstone and Rockhampton, Queensland. Geological 
Horizon: The Narrows Tertiaries. 
Description : Carapace small, in lateral view ovate to elliptical and 
subreniform, highest at about middle. Height rather greater than one- 
half of length. Dorsal margin gently and evenly arched. Ventral 
margin practically straight. Anterior extremity subacute, somewhat 
truncated dorsally. Posterior extremity more evenly rounded. Convexity 
of valves moderate, nearly regular, but rising rather more steeply from 
dorsal margin than from ventral. Position of greatest thiekness of 
carapace located centrally. Valves subequal, left slightly overlapping 
right along ventral margin and the two extremities. Hinge somewhat 
thickened, occupying approximately central third of dorsal margin. In 
profile acute-ovate. In end view oval. Surface of valves delicately 
ornamented with elliptical riblets, reticulating to some extent, but 
generally concentric about apices of the two valves — i.e., the position of 
greatest thickness of carapace. Ornament somewhat resembling minute 
plicae or papillary ridges of human finger tip. Shell thin, fragile, ehitino- 
calcareous, and transparent to translucent. 
Dimensions of holotype: Length, 043 mm.; height, 0*25 mm.; 
approximate thickness of carapace, 0.2 mm. 
Remarks: A search of available literature has failed accurately to 
place this species generically. In ornamentation and general shape, 
however, it resembles some species of the recent genus Ilyodromus , and 
it is here tentatively referred to that genus. However, the carapace is 
not so compressed and is much smaller than any of the recent species of 
this genus, so it may be generically distinct. It resembles most closely 
Ilyodromus ellip\ticus Sars, but differs in having the dorsal margin more 
strongly arched, the ornament more conspicuous and concentric instead 
of generally longitudinal, and in its much smaller size. It also shows 
some resemblance to Ilyodromus obtusus Sars, but here again the 
differences are quite marked. Both these recent species of Ilyodromus 
have been recorded from New South Wales (Henry, 1923). 
This striking and easily recognised ostracod is moderately short- 
ranged, and a zone in The Narrows Tertiaries has been based on its 
proved vertical range of 115 feet — the Zone of Ilyodromus f concentricus. 
Through this interval the species has clearly been observed to increase 
gradually in abundance from very rare to very common, and, after 
reaching its acme, to decrease gradually until it becomes extinct. 
However, although it is very common in some of the samples from The 
Narrows, relatively few undamaged valves and complete carapaces have 
been found. This is apparently due to the very delicate and fragile 
nature of the shell and the fact that the valves are easily separated 
after death, since the hinge is edentulous. The outstanding characteristics 
of this species are the ornamentation and the small size. Some variation 
in outline within the species has been observed, which does not appear to 
be the result of crushing. While in most individuals the ventral margin 
