259 



"the track. His species was evidently described from a young 

 example, in which the columella was partially at least worn 

 away by fossilization. He subsequently discovered the rem- 

 :jiants of a columella in the species, and then established a new 

 genus, Koilotrochus, for its reception, in the diagnosis of which 

 he remarks : — "Columella rudimentary confined to a few papil- 

 lary projections at the base of the deep and wide calicular 

 fossa." Some corals collected by Mr. T. S. Hall and myself, 

 also from the Muddy Creek beds, are, I consider, adult examples 

 of Woods' species, but they possess a conspicuous papillary 

 columella, which, combined with other characteristics, places 

 them in the genus Platytrochus. A few young immature indi- 

 viduals accompany the adult ones in our gatherings, which, in 

 regard to the corallum, resemble Woods' figure. Their septa 

 also are arranged on the same plan as in his illustration of the 

 calice, but instead of a vacant central space there is the usual 

 -columella of the species. When describing the calice, Woods 

 gives its diameters as 3 mm. and 1.5 mm. respectively, or in 

 the ratio of 100 to 50, but in his enlarged drawing the ratio 

 -of the axes is as 100 to 66 ! The species name is certainly 

 an unfortunate one, as it contradicts an essential characteristic 

 of the coral, but in accordance with common usage must be 

 retained. 



I redescribe the coral from an adult well preserved example, 

 and also supply illustrative drawings. 



Platytrochus vacuus, T. H'oods (sp.). 



PI. v., figs. 4o, h. 



1877. Smilotrochus vacuus, T. Woods, Proc. Roy. Soc, New 

 South Wales, vol. xi., p. 190, pi. ii., figs. 2, 2a. 



1878. Koilotrochus vacuus, T. Woods, Proc. Linn. Soc, New 

 South Wales, vol. ii., pp. 313-4. 



Corallum small, wedge-shaped, and much comipressed. The 

 edges are nearly but not quite straight until inferiorly they 

 bend at a sharp angle, and almost meet at the broadly pointed, 

 projecting base. The flat surfaces slope uniformly downwards. 



Calice sub-plane and elliptical with its major axis double the 

 length of the minor. Septa slender, granulate, and in six 

 systems, which show the same cyclical arrangement as in P. 

 Airensis. They are subequal in the central systems ; also sub- 

 equal but generally smaller in the end systems, except the two 

 extreme primaries, which are the most prominent septa in the 

 ■calice. The papilli of the columella resemble those in the 

 three other species described. 



