215 
P achy doniUS , Iccvis, J. Morris, 1855, in Strzelecki’s Phys. Descr. N. S. Wales and V. D. 
Land, p. 272. 
,, ,, J. D. Dana, 1849, Geol. Wilkes’ U.S. Explor. Exped., p. 691. 
This is a large, elongated, oval shell, with relatively shallow^ valves, 
and moderately thick, slightly recurved, snbmedial heaks. The anterior 
margin is rather regularly rounded, while the posterior is slightly angular 
and depressed. The external surface is ornamented with some irregular, 
concentric undulations, especially towards the ventral margin ; the entire 
surface is also covered with fine striae of growth. Although the shell is 
extremely thin there is no trace of the external ornamentation on the interior 
of the valves which is perfectly smooth. 
Dimensions. — The length is about ten, the breadth seven, and the 
tliickness six centimetres. 
Delations and Dijferences. — This species is easily distinguished from 
the preceding by its much less globose form, its small relative thickness, and 
above all by the thinness of its test. 
Horizon and Localities . — This was found at Illawarra by Mr. J. 
Morris, and at Harper’s Hill by Mr. J. D. Dana; and in a greyish, rather 
compact limestone at Lochinvar, associated with P. glohosus. 
Pachydomus gig as, F. McCoy. 
PachydoniUS gigas, E. McCoy, 1847, Ann. Mag. Nat. Hist., XX, p. 301, pi. 16, fig. 3. 
This shell is rather large, elongated, and oval, very nearly one-third as 
long as broad, with moderately deejt valves. The beaks are thick and recurved, 
and placed towards the anterior third of the shell. The posterior margin is 
slightly truncated, with rounded angles ; the anterior margin is rounded, 
rather short, and united to the ventral margin by a slight concave sinuosity. 
The whole surface is ornamented with slight undulations and concentric striae 
of growth. The test appears to have been very thin ; but it must have been 
smooth inside as no traces of the external ornamentation have been preserved 
on the internal casts of the valves. 
Dimensions, — The length is fourteen, and the breadth nine centimetres. 
