272 THE ATRYPIDAE OF NEW SOUTH WALES, 



06s. — The features which separate this genus from Atri/pa are the great 

 bicouvexity and smoothness of the valves, absence of marginal fringe and radial 

 striae or ribs. 



Genotype, Atrypoidea australis. 



Atrypoidea australis n.gen. et sp. (PI. xiv., tigs. 1-18; PL xv., figs. 8, 9; 



PL xvi., figs. 7, 13.) 



Shells intensely biconvex to subglobular in mature specimens. In young 

 specimens mildly convex beak, as maturity is approached the convexity and front 

 sinus of the valves rapidly develop, and at no stage of growth is the difference 

 in the convexity of the two valves of this species very pronounced, though that 

 of the brachial valve is the greater: cardinal angles rounded, hinge line wide, 

 straight or only gently arcuate. Pedicle valve has, in some large specimens, a 

 faint medial fold extending from the umbo to the front and, on each side of this 

 fold, a very shallow faint sulcus; opposed to this in the brachial valve is a faint 

 sulcus bounded by an exceedingly faintly defined fold, but this feature does not 

 appear to be constant, for, in some forms, the plainness of the surface is unin- 

 terrupted and in others a feeble sulcus takes the place of the fold; front sinus 

 wide and deep in mature specimens, its intensity gradually develbping with age, 

 ^ery immature shells having none. Brachial valve very convex and at no stage of 

 growth showing a decided fold, beak small and concealed. Dimetisions of a ma- 

 ture individual (PL xiv.. figs. 5 and 6) : Length, 28, width, 28, thickness, 22 mm. 

 The projiortions of these measurements remain very constant for sizes of shells 

 from half to full maturity. 



06s. — A specimen of this species was described l)y one of us* under tiie 

 name of Merlstina australis. This determination and description was based upon 

 superficial features only, which indeed very closely resembled those of the Meris- 

 ti)ia group. That there were good reasons for this determination is shown by 

 the fact that specimens of this new group, showing only external features, were 

 submitted to British palaeontologists and they referred them to Meristina tumida. 

 More recently, specimens of the group with spirals preserved enable us now to 

 place it in or near its proper phylogenetic position, and that it belongs to 

 Atnjpidae, we believe, cannot be disputed, though certainly a few features of its 

 internal structure remain to be revealed, but we do not expect the revelation of 

 them will materially alter the views we have arrived at with respect to its classi- 

 fication. Externally the group shows some features rot seen in Atrypa, yet 

 on the other hand has others that are truly Atrypoid, as, for instance, the straight 

 hinge line, absence of cardinal area, high rounded cardinal angles, d?nressed in- 

 curved beak of the pedicle valve, and very strongly convex biachial valve. They 

 are certainly extraordinary Atrypids and up to the present are known to occur 

 only in the limestones of Molong and the impure limy shales of the Bowning 

 Series, and the specimens in these different districts are, for the most part, alike 

 specifically. Those belonging to the present species from IMolong are uniformly 

 of much larger size than the Bowuing-Yass (Hatton's Corner) representatives. 

 Tliis variation may have arisen from more favourable conditions for their de- 

 velopment having prevailed in the Molong area than at Bowning. In the former 

 the sea was clear, and free from the muddy sediment present in the latter. Be- 

 sides the relative smallness of the Hatton's Corner members of the species, they 



•Dun. Records Geol. Surv. N.S.W., vii., 1904. p.318. 



