694 APPENDIX L 
Plate 5, fig. 2, natural size—The form of this species is subrotund, a little oblong. 
Length of one specimen 3,/; inches; height 2°, L.; thickness 55°, L. The same propor- 
tions are represented in Sowerby’s figure. The palleal impression, in the specimen above 
referred to, is nearly an inch from the lower margin. The flanks are flattened, and 
straighter than in the ewneatus. The surface of the cast, from the beak downward, is 
flattened as in the cuneatus. ‘The valves are less than half a line thick in the most pro- 
minent part, and just exterior to the palleal impression are nearly two lines thick. The 
specimens have the shell calcareous, but with the cross cleavage of calc spar. 
Megadesmus antiquatus, J. D. Sowerby, Mitchell’s Australia, vol. i., pl. 3, fig. 2, pp. 15, 16. 
Pachydomus antiquatus, J. Morris, op. cit., p. 272. Wollongong is mentioned as a locality of 
this and the preceding species, the correctness of which we much doubt. 
34. Pacnypomus tevis (J. D. Sowerby), Morris.—One of our specimens from Har- 
per’s Hill, resembling in general characters the antiquatus, has nearly the proportions 
of Sowerby’s /evis. Its length is 3 inches, and breadth 2g inches (= 74, L.) 
Megadesmus levis, J. D. Sowerby, loc. cit., pl. 3, fig. 1. 
Norre.—The Megadesmus globosus of Sowerby, (loc. cit., pl. i.) is another species 
from Harper’s Hill, which we did not meet with. It is represented with the very thick 
valves characteristic of this genus, in which particular it differs widely from the Wollon- 
gong species, which it externally resembles, and which has apparently been here referred. 
It has nearly the proportional length and height of the antequatus, but in Mitchell’s figure 
is nearly as thick as it is high. 
Genus M/KONIA (Dana).—Slightly inequivalve ; oblong, elliptical, or sub-ovate, 
larger anteriorly, gaping a little or not at all. Beak moderately prominent. Liga- 
ment external. Muscular impressions three ; the large anterior subovate, and somewhat 
pointed above ; the small anterior, facing the same way with the larger, the muscle having 
a lateral instead of forward direction. Palleal impression, distinct or indistinct ; posteri- 
orly, rather abruptly bent upward to the posterior muscular impression. Lateral surface 
usually compressed, and in some species strongly concave. Flanks often broadly flat- 
tened. Beaks of cast approximate at apex, and acuminate, terminating often in an 
acute or thin apex, and sometimes a slender process, 
The general appearance of a cast of some species of this genus is much like that of the 
Pachydomi. This is strikingly the case in the exterior form, and the palleal impression. 
But the unequal valves, the form of the large anterior muscular impression, and the 
reverse position of the smaller anterior, as well as the approximated and acuminating 
apices of the beaks of a cast, require their separation. 
In a brief notice of the Expedition Fossils in the American Journal of Science, vol. iv. 
ii. series, the species here included under Meonia are referred partly to other genera ; yet 
as they all agree in the character of the anterior muscular impressions, and the transitions 
in form are quite gradual, they are here brought together. The divisions may, perhaps, 
form sub-genera, as there are some striking differences, as follows : 
Monra.—An accessory excavate muscular impression, about as large as the smaller 
anterior, situated anteriorly near the summit of the beak. Palleal impression strong. 
[Species large. ] 
