25 
It is tlie 
1. Deltopecten illaioarensis^ Eth. fil/ — Specimens obtained at the 
Mount lE’itton Gold-field, l)y tlie Queensland Geological Survey, formed 
the type of the genus Deltopecten. The hinge-plate was ol^served to be broad 
and strong, with coarse resilium furrows and a deltoid chondrophore. The 
surface of some of tlie valves presented a ventral lateral flattening. 
Tiie limitation of Deltopecten illmoarrensis, of all tlie Australian 
Palmopectcns, has caused us the greatest trouble. If Morris’ figure represents 
a ])art of a flat valve, as it appears to do, the figures of De Koninck cannot 
possibly represent the species, and by analogy that of Dana also, for it is 
morally certain that the two are identical. "VVe are further hampered by the 
question of locality, as already explained under Aviculopecten Mitchelli, 
nobis. Were it not for the fact that the type is believed to be in existence, 
we ivould feel much inclined to reject the name in toto, as the simplest way 
out of a specific complication. The whole matter turns on a very simple 
question : What is “ Decten Illmvarensis,'^ Morris ? 
The shell we have assumed to be this species is orbicular, lii-convex, 
but iuequivalve, and to some extent tumid. In consequence of the 
imperfection of the auricles in all the specimens before us, it is difficult to 
distinguish the right from the left valves ; we therefore, in this case, use the 
terms convex and less convex valve, but we believe the former to be the left, 
and the latter the right valve. In the convex valve one of the slopes 
(anterior ?) is abrupt and steep, cutting off an apparently rectangular auricle ; 
a similar auricle is present in the less convex valve, but in none of our 
specimens are the opposite auricles of either valve preserved ; the umbo 
appears to have been obtusely pointed. The surface of this valve is uniformly 
convex from the umbo to the ventral margin, and traversed by from twenty- 
five to thirty-five thick, rounded, distant costse, with here and there a smaller 
interpolated rib, the radii on the quadrangular (?) auricle being numerous; 
the intercostal spaces are wide and flat. 
In the less convex valve the surface is convex from the umbo forwards, 
until approaching flie ventral and latero-ventral regions, when a decided 
flattening, or even slight concavity, sets in, like that of D. limeeformis. The 
cosbc are less numerous, from twenty to thirty, and more angular than in 
the convex valve. In both the costm and intercostal spaces are crossed by 
* Etheridge, Junr. — Gicol. Pal. Q’land, &e., 1892, p. 269, t. 41, f, 3, t, 43, f. 2. 
