13 
Aviculopecten tenuicollis, Dana, sp. 
(Plate XTTT, Fig,s. 10-12 j PI. XIV, Fig. 5.) 
DgcIgU tGnuicollis, Dana, Ain. Journ. tSci., 1847, iv (2), p. 160. 
Dccten tpnuicollis, Dana, Wilkes U. S. Ex))lor. Fxpecl., 1840, X (Geol.), p. 705, t. 9, f. 7, 7rr. 
Observations . — Dana ligiired an imperfect left valve, the anterior auricle 
broken, the posterior wanting. lie described it as siiborbicular, with twenty- 
four slender, subacute, smooth costm, intermediate secondary costae, and flat 
intercostal spaces. It is the — 
1. Aviculopecten tenuicollis, fil.^ — Several specimens were described 
from the Pindan Sands of the neighbourhood of Derby, North-West Australia. 
They present the general features of the species, supplemented liy the presence 
of latilamina?. 
2. Aviculopecten tenuicollis, Eth. fil.^ — Additional exam]des from 
Western Australia indicate that in addition to the latilaminm the costm 
were intersected l)y fine, close, flat, frill-like laminae, rendering the costa; 
slightly spinose. 
We find it impossible to confine our determinations strictly within 
Dana’s definition of his species. He speaks of “the specimen”; we 
have several before us. We find revolving around specimens similar to 
Dana’s figure, a number with the same outline and same form of costa', but 
differing amongst themselves in the number of the latter, and to some extent 
in the length of the posterior auricles ; these we regard simply as varieties. 
The siiborbicular or scalloji-shaped left valve is convex, equilateral 
(omitting the auricles), the anterior and posterior slopes converging rapidly 
to the umbos, the anterior slopes sharp, and clearly defining the auricles from 
the bodies of the valves ; the umhos acute, and the dorsal margins long, but 
barely as wide as the valves. The anterior auricles are large, deeply 
triangular, flat, and with a rounded outer margin, not separated off by a 
byssal sinus, but simply by an emargination. The posterior auricles are 
elongately triangular, with sigmoidally curved outer margins, but tliey 
' Etheridge, Juiir. — Proc. Linn. Soc. N. S. Wales, 1889, IV (2), p. 20,3. 
^ Etheridge, Junr. — Bull. Geol. Survey W. Austr., 190.3, No. 10, p. 23, t. 3, f. 3 and 4. 
