BY JOHN MITCHKM.. 549 



minent, beak small, always visible. Hinge line straight or very mildly artiiate, 

 cardinal angles rounded. The septa are short. 



Dimensions of adult individuals. — 



Length 10.5 mm. Width 13.5 mm. Depth. 8.3 mm. 



„ 8.3 „ 12. „ 7. 



„ 10.4 „ 13. „ 8.3 



. 10.4 „ 12. „ 7.8 



These measurements are fairly proportional. 



Obs. — In some respects this species resembles youthful individuals of B. 

 linguifera var. u-ilkiiisorii Eth. Jr., and is suggestive of having been derived from 

 t!)at species by the arrest of its development in the early stages of growth, but 

 at no stage of development can the one form be mistaken for the other. Dimen- 

 sionally they are widely different, — the one seldom reaching a length of 10.5 

 mm. and only rarely exceeding 13 mm. in width, while the other may reach 25 mm. 

 for each of these dimensions. The width in tlie Molong species is always much 

 {Teater than the length, but in the other the width only very slightly exceeds the 

 /ongth. This fossil occurs in clusters, numbering hundreds of individuals occa- 

 sionally in a cluster in a massive bed of grey limestone associated with Atrypoidea 

 australis Mitch, and Dun, A. angusta Mitchell and Dun, Leptaena rhomboidalin 

 Wilckens, etc. 



Loc. and horizon. — Some eight miles west of Molong, Parish of Bomey, 

 ("ounty Wellington. Probably Upper Silurian. 



SlEBKRELLA GLABRA, n . Sp . (Plat* XXxi., flgS. 13-15.) 



Spec. Chars. — Shell smooth, thick, subpentagonal or subquadrate according 

 to stage of growth, concentric lines faintly visible on some specimens. Pedicle 

 valve very convex, umbo very tumid and prominent in adult specimens. Beak 

 acutely pointed, incurved and strongly depressed on to that of the brachial valve. 

 Fold only moderately conspicuous, originating just in front of the umbonal region 

 and terminating with a straight edge at the anterior sinus. Hinge line wide, car- 

 dinal angles high and rounded. Brachial valve only moderately convex in the 

 posterior half, laterally flat to subeoncave; sinus wide, shallow and moderately 

 indenting the opposing valve. 



Dimensions (adult specimens). — Length, 21, 21.9 mm.; width. 21, 20.8 mm.; 

 depth, 16.5, 13.0 mm.; (specimen of medium growth), length 15.6, width 18.7, 

 depth 8.8 mm. 



From these measurements the development of the shell would appear to have 

 been very variable in different individuals, or rather at different stages of growth. 



06.*.— This shell is different from the old Sieherella (Pentamerus) ijaleatus 

 in several particulars, so evident that their enumeration is unnecessary. The 

 radial ribbing characteristic of the genus is practically absent fi;om the Australian 

 species, for the only traces found on it are very faint folds on each side of the 

 sinus anteriorly, and an equally faint and hardly visible fold on the medial part 

 of this sinus. 



Specifleally, as far as my knowledge enables me to judge, the species here 

 described has no very close relation among the species occurring in Europe and 

 America. 



Up to the stage of medium growth, the umbo of the pedicle valve of .?. 

 glabra is not prominent, and the beak does not overhang that of the brachial 



