NKWTON : OPALIZED SHELLS OF NEW SOTJTH WALES. 231 



fossils makes them important for reference. The more complete, so 

 far as contour is concerned, and which was found at Lightning Ridge, 

 exhibits both valves in the closed state, the margins of which are 

 generally well defined, while tlie anteriorly situated summit region 

 is very depressed, the nmbones having been completely eroded away, 

 the postero-dorsal chaiacters being also without proper definition. 

 The shell-structure is only very parti.illy preserved, altliough sufficient 

 remains to show the presence of concentric growth-lines and some 

 obscure microscopical radial striations. The second example is from 

 White Cliffs, also with closed valves, and is larger than the previous 

 form. In this the umbonal regions have disappeared tlirough erosion, 

 and a fractured posterior end of the specimen prevents an exact 

 knowledge of its original length, although this seems to have been 

 about 56 mm. The dorsal view, however, exhibits a part of what 

 would have been a fairly long lanceolate escutcheon with sharply 

 angulate lateral borders, enclosing a well-rounded, lengthy ligament, 

 bearing annulations of growth, and furnished with tapering ex- 

 tremities, the concentric growth-lines forming a fairly sharp angle 

 where they meet the Tnargins of the escutcheon. Tlie radial striations 

 are either absent or only very obscurely traceable, erosion having 

 probably obliterated these finer details of sculpture. 



1 would wish to associate with this shell the name of Mr. J. B. 

 Jaquet, who was the first geologist to describe the Lightning llidge 

 opal deposits, and who previously had furnished important details in 

 connexion with the constitution of the beds at White Cliffs. 



Localities. — Liglitning liidge and White Cliffs, New South Wales. 



Collections. — British Museum (Geol. Dept., L. 21833) ; llev. F. St. J. 

 Thackeray. 



TJnIO WHITE-CLIFFSENSIS, U.Sp. PL VI, FigS. 7, 8. 



Description. — Shell of small size, with moderately inflated valves, 

 length about li- times the height; umbonal regions anterior, coarsely 

 rugose or marked with strong, widely V-sluiped costae ; anterior 

 margin rounded, posterior side with an elongate, abrupt, oblique, and 

 narrow, angulate, ridged area, in front of which the valve is slightly 

 excavated. Sculpture beyond the V-shaped costal rugosities of tlie 

 umbonal area consists of periodical growth-divisions, and numerous, 

 closely set, microscopical concentric striations, which at the posterior 

 ridge become angulate, and take an upwardly oblique direction on 

 the surface of the posterior area. 



Dimensions [with closed valves). — Length 22, height 15, diameter 

 10 mm. 



Remarks. — This specimen exhibits a pair of closed valves, which, 

 liowever, are not (j^uite in situ, having slightly shifted from each other 

 during the process of fossilization. It is a well-marked form, with 

 all the characters referred to properly defined ; the umbones themselves 

 are not present, but the rugose umbonal regions are quite definite and 

 characteristic of Unioniform shells. 



Locality. — White Cliffs, New South Wales. 



Collection. — Eev. F. St. J. Thackerav. 



