90 Proceedings of the Royal Society of Victoria.. 



ribbed surface. Umbo incurved and overhanging the ligamental 

 area, which is a well-defined isosceles triangular space, the base 

 of which is just about half the width of the hinge. Hinge 

 furnished with from 22 to 26 oblique and angular teeth, most 

 usually 12 on each side, with a tendency for the medial ones to 

 become obsolete in the extreme adult. Surface closely covered 

 with broad radial ribs, ranging from about 30 to 35 in small 

 specimens, up to about 50 in the adult, ribs slightly convex, with 

 very narrow, almost lineal, interspaces in young shells, but in the 

 adult the ribs are decidedly flattened; the ribs are closely, finely, 

 and regularly radially striate, each rib bearing near the ventral 

 margin ten striae, anteriorly and posterioi'ly the ribs become 

 obsolete, but the radial striae are present, and m'ucli stronger 

 than on the ribs. The radial sculpture is crossed by fiine concen- 

 tric lines of growth. Interior of valves strongly denticulate along 

 the ventral margin, bearing about 25 strong denticles, running^ 

 about 8 to 10 in 10 mm., thence both anteriorly and posteriorly 

 diminishing in size, but extending right up to the hinge. 



Variety, intePmedius, var. nov. (PI. XIV., Figs^ 10, 11). 



A variation of tlie above shows a less orbicular outline with 

 sloping shoulders and consequently an apparently more promi- 

 nent umbonal region, and with coaser radial ribbing, 25 to 28- 

 being about the average number. 



Variety, paucicostatus, var. nov. (Pi. XIV., Fig. 12, 

 and PI. XV., Fig. 9. 



Another form wliich appears but a variation of the above 

 species, is intermediate in shape between it and the foregoing 

 variety, l)ut rather closer related to the latter, being distin- 

 guished most readily by the still coarser radial ribbing, as it 

 bears only about 20 strong, convex ribs, neglecting the obscure 

 and ill-defined ones on the anterior and posterior slopes. 



Dimensions. — Type, antero-posterior diameter, 42 mm.; umbo- 

 ventral diameter, 44 mm.; thickness through one valve, 18 mm. 

 Others range from 24 by 22, 21 by 20, 18 by 17, 17 by 16, 12 by 

 11, to 9 Ijy 9 and 8 by 8, and smaller. Var. iiifeenuiedius, antero- 

 posterior diameter, 21 nnn., uml>o- ventral <lH;imeter, 20 mm.,. 



