84 Proceedings of the Royal Society of Victoria. 



a slight convexity. The tergolateral margin is boldly curved, so 

 that the part near the basal margin is almost at right angles to 

 that near the occludent one. The basal margin bears a slightly 

 thickened ridge within, and there is no umbonal tooth. Seen 

 from the peduncular side, the basal margin is moderately curved. 

 The lines of growth show that a slight addition was made to the 

 basal margin throughout life. Radial 'markings are absent. 

 Length of occludent margin of type, 30 mm. ; greatest width of 

 valve at right angles to this line, 27 mm. A specimen from 

 Spring Creek, collected by Mr. G. B. Pritchard, measures 38 mm. 

 in its greatest breath. 



Locality and Age. — Waurn Ponds, quarry below the school 

 (type). Spring Creek (G. B. Pritchard). Jan Jukian (? Eocene). 



I think there can be no doubt of the correctness of the generic 

 position to which I have referred this fossil, though in shape it 

 differs somewhat from the corresponding valve in all the recent 

 species. The absence of an umbonal tooth may be due to 

 corrosion, which is a fault many of our fossils from the 

 polyzoal rocks exhibit. Still, if originally present, the tooth 

 must have been small, and it is of course absent in some recent 

 species. 



Another peculiarity is the bending of the growth lines along 

 the basal margin, a feature which Darwin says is not shown by 

 any of the species known to him, for no addition by growth is 

 made in this position by recent forms. 



As regards the age of the specimens, no one apparently 

 would suggest that the Waurn Ponds beds are younger than 

 Miocene, while some Australian authors call them Eocene. Even 

 if we take the youngest estimate, namely Miocene, the fossil is 

 the oldest member of the genus yet discovered. 



EXPLANATION OF PLATE XI. 



Figs. 1, 2, 3. — Melitodes (?). Stem-joints. Grange Burn. 

 „ 4, 5. — Pentagonaster, sp. Marginal plates seen from 



from two directions. Waurn Ponds. 



