218 
'Relations and Differences. — By its small size and great thickness this 
species differs from all its congeners. 
Horizon and Localities . — According to McCoy it is common in the 
Wollongong sandstone. 
Bachydomus politus, J. D. Dana. 
PL XIX, Fig. 4. 
A-Startila polita, J. D. Daua, 1S49, Am. Journ. Sci., IV (2), p. 155. 
„ ,, Idem, 1849, Geol. Wilkes’ U.S. Explor. Exped., p. G90, pi. 4, fig. 2. 
This is a moderate-sized, oval shell, rather longer than broad, with 
slightly recurved beaks, placed in the front quarter of the shell. Its anterior 
and ventral margins form only one regular curve, while the j)osterior is 
slightly truncated. The exterior surface is smootli and glossy, and shows 
some concentric lines of growth. The shell is very thin ; the muscular 
impressions are not distinct ; the anterior impression is rather large, and oval, 
and striated transversely ; the posterior impression is so faintly marked as to 
he almost imperceptible. 
Dimensions. — The length is thirty-nine, breadth thirty-one, and thick- 
ness twenty-two, millimetres. 
Relations and Differences. — In form this species is somewhat like 
P. oralis, McCoy, from which it differs, not only in its proportionate 
dimensions, but by the thinness of its test, and the gloss of its external 
surface. 
Horizon and Localities . — Mr. J. D. Dana found this sjiecies at Black 
Head, Illawarra, and Mr. B. Clarke at Wollongong. 
Bachydomus Danai, L. G. de Koninck. 
PI. XIX, Fig. 5. 
This is a medium sized, oval or slightly cuneiform, gibbous shell, as 
long as broad, with small anterior beaks, approaching close to each other. 
The hinge-line is arched ; the external ligament must have been long ; there 
is no lunule. The external surface is covered with a series of flat concentric 
folds, not imbricated, and separated from each other l)y narrow, well-marked. 
