89 
where they are parallel. They are separated from one another hy narrow 
hut pretty Avell-niarked furrows ; their surface is rugose. The hinge is com- 
jiosed of ten posterior teeth, the last live of which are much more developed 
than the first, and about twenty anterior teeth, of Avhich those farthest 
distant from the heats are also a little stronger than the others. No inter- 
ruption can he seen in the dental series, and one cannot see in it the least 
trace of a ligament pit, which constitutes one of the chief characters of the 
genus to which the species belongs. The ventral border is sharp and smooth. 
Dimensions . — Length, eleven millimetres; width, eighteen millimetres ; 
thickness, seven and a half millimetres. 
Delations and Differences . — This species has some resemblance to 
T. {Dullastra?') antiqua, Sowerby, which it resembles most in the surface 
ornamentation. It may, however, be easily distinguished from it by its more 
oval form and the greater number of concentric ridges Avith which the surface 
is covered, and liy its relatively greater thickness. 
Horizon and Localities. — A single well-preserved specimen of this 
species has been discovered in the blackish argillaceous limestone of the Yass 
District. 
AVICULOPECTEN, McCoy. 
1. Aviculopecten Etheridgei, L. G. de Koninck. 
PI. Ill, Fig. 9. 
I am acquainted with the left valve only of this pretty species. With 
the exception of the ears, it is almost regularly circular, and almost as long as 
broad ; it is fairly uniformly but very slightly arched ; the test is very thin ; 
the surface is ornamented Avith about forty very narroAV radiating ril3S, the 
width of which is little different from one end to the other ; they all have 
their origin in the beak and undergo no bifurcation ; the distance apart has 
no regularity; the remaining ribless part is flat and almost smooth, some 
slight concentric undulations only produced by the successive growth of the 
shell is all that can be seen. The anterior ear is subtriangular and separated 
from the rest of the valve by a Avell-marked furrow ; the ligamental margin is 
perpendicular to the principal axis of the shell ; the surface is ornamented 
with three or four small radiating ridges, of which the principal helps to form 
