BY JOHN MITCHELL AND W. S. VUK. 273 



also appear to have a slightly less conspicuous umbo and beak than those from 

 Molong; also the strong tongue-like anterior sinus would seem to have developed 

 at an earlier stage of growth in those from Hatton's Corner than in the others. 

 Further, in one specimen of this species from Molong, the braehidia show a rather 

 strong droop apically, but this would appear to be accidental, for others from 

 Molong show the apices of the spiralia to be directed almost towards the centre 

 of the brachial valve, and this agrees with the Hatton's Corner types. 



Loc. and horizon. — Near Molong, Parish of Bomey, County Wellington; Hat- 

 ton's Corner, Parish of Yass, County Murray; Gurnett's Selection, three miles 

 west of Bowning, Parish of Bowning, County Harden. Upper Silurian. 



ATRYPOinEA ANiiUSTA, n . sp . (PI. xiv., figs, 20-29.) 



Shell intensely biconvex, subquadrate, smooth; length greater than width, 

 valve margins intensely sinuate, front sinus very deep. Hinge line mildly ar- 

 cuate; umbonal ridges low and spreading. Pedicle valve transversely and longi- 

 tudinally strongly convex, just below the umbonal region the inflation is so great 

 that it gives the shell quite a hunchback aspect; towards the front a very faint 

 sulcus is developed, the sides of it being slightly more depressed than the medial 

 portion, indenting lip tongue-like and long. Umbo of moderate size, beab 

 strongly incurved and depressed, aperture small. Brachial valve very much 

 arched transversely, highest in front of its centre, and developing into a strong 

 fold on the anterior third. Dimensions: Length, 23.4, width, 22, thickness, 18.7 

 mm., for the largest specimens from Bowning and Molong; but the specimens 

 from Hatton's Corner so far collected, are much smaller, the largest from here 

 measuring— length, 19, width, 17.2, thickness, 14 mm., respectively, and an im- 

 mature specimen from Molong had the same dimensions. The relative propor- 

 tions of these measurements are fairly constant for the specimens from all three 

 localities". . 



(Jbs. — The internal structure of the species has been observed in the Bowning 

 form only and agi'ees in the spiralia with the genotype. From the foregoing 

 species the present differs in being much more biconvex, having a less conspicuous 

 umbo, more strongly incurved and depressed beak, narrower hinge line, greater 

 length than width, a greater relative thickness, much stronger sinuosity of the 

 lateral and front margins. 



It may be noted that very immature shells of this species cannot be dis- 

 tinguished from similar ones of A. australis. or at least we have not been able to 

 do so up to the present. 



Loc. and horizon. — The same as for the preceding species. 



References to records of Atrypa from other States of Australia. 



Queensland. 



Atrypa reticularis Linnaeus. — The occurrence of this species has been re- 

 corded from the Fanning River, Burdekin Downs, by the late R . Etheridge Jun . * 

 tnd by Foord.f In 1892, R. Etheridge repeated his previous records.! 



Through the courtesy of Mr. B. Dunstan, Chief Government Geologist of 

 Queensland, we have been enabled to inspect the original specimen referred to 



•Proc. E. Phys. See. Edinh., v., 1880, p.270. 



tGeol. Mag., vii., (3), 1890. p. 100. 



JGeol. Pal. Qld. and N.Guinea, 1892, p.05, PI. 4, t. -t. . 



,^'A.»^-S:y 



