240 ON SOME xMESOZOIC FOSSILS FROM CENTRAL AUSTRALIA, 



unequal, radiately ribbed, the anterior one in the form of an 

 elongated triangle, produced along the dorsal margin for half the 

 length of the shell, posterior smaller, fan-shaped. 



This small, smooth, depressed Perten, belongs to a type which is 

 common in the mesozoic rocks, the middle mesozoic especially. 

 In a genus so abundantly represented in species during almost every 

 geological period resemblances can be found on every side. But it 

 is particularly like P. socialis, Moore, which is a sub-orbicular shell 

 moderately convex, umbones convex and pointed, auricles unequal, 

 anterior one much the largest. Mr. Moore says that the external 

 ornamentation of the shell is not well preserved, but it appears to 

 have been nearly smooth, without visible concentric striae but with 

 depressed radiating ribs. In general form it is not unlike 

 P. rigidus. * It is one of the most abundant shells in the boulders 

 from Wollumbilla, many specimens of various ages appearing on 

 their fractured surfaces. 



The fossil I have described might well be a young state of Mr. 

 Moore's fossil. It is much smaller, the average dimension from 

 hinge to margin is about 6 millimeters with a tranverse diameter 

 of 5. It is found abundantly scattered over some fragments of dark, 

 olive greenish marl, looking not unlike Nummulites. As I have 

 only seen the figures and descriptions of P. socialis, and as the 

 details are very imperfect, I think it better not to make too hasty 

 an identification but point out the resemblance for future enquiry. 



Avicula bwklyi, Moore, vel reflecta, Moore var. gilliatti, pi. xii., 

 figs. 4, 5 and 6. The great mass of the fossils in this collection is a 

 large species of Avicula of which a figure is given. It possesses all 

 the characters of both the fossils above-named in different specimens 

 and I think that the specific distinctions between the two will be 

 found on examination to disappear. Any one who examines the 

 series figured by Mr. Moore from Wollumbilla including eight 

 species, will be inclined to refer them all to one, differing from each 

 other merely in size and mode of growth. The posterior wing 

 auricle is lengthened and undulating in the large and old specimens 



* A fossil species of the Great Oolite, Wiltshire. See Sowerby's Mineral 

 Conchology, plate 205, fig. 8. 



