22 
2. Lept^na compressa, J. de C . Soiverhy . 
Orthis COtnpressci, J. de C. Sowerby, 1889, in Murchison’s Silurian System, pi. 22, fig. 12. 
Strophoilienci cotnprcssa, J. Phillips and Salter, 1848, Mem. Geol. Survey Gt. Britain, II, 
p. 379. 
,, ,, Salter, 1859, in Murchison’s Siluria, p. 100, fig. 7, and pi. 9, 
fig. 16. 
,, ,, T. Davidson, 1871, Mon. Brit. Sil. Brach., p. 311, pi. 46, fig. 7-10. 
Shell wider than long, subsemicirciilar or suhrectangular. Ventral 
valve feehly bnt fairly regularly arched ; area narrow, fissure small and 
covered by a pseudodeltidinm. Dorsal valve almost fiat or slightly concave. 
Surface ornamented with a great number of small radiating ribs. 
Dimensions. — Length, twenty-two millimetres ; breadth, thirty milli- 
metres. 
Horizon and Localily.- — This species is usually associated with Atrypa 
hemispherica, and they have both been found at Duntroon. 
Genus — STROPHOMENES, Dajinesque} 
1. Strophomenes pecten, Linneeus. 
Anomia pecten, Linneeus, 1767, Syst. Nat., Ed. XII, I, pars, ii, p. 1152. 
Stropliomena pecten, Davidson, 1871, Mon. Brit. Sil. Brach., p. 304, pi. 43, fig. 1-11 
This species is easily recognised by its semicircular outline, by the 
compression and slight convexity of the valves, and by the ornamentation of 
their outer surface. This ornamentation consists of a large number of 
radiating ribs, at first fairly equal to one another, but as the shell grows 
one or two new ribs are developed between the first, and make these appear 
thicker, thus producing, in relief, a very marked alteration. On good 
specimens these ribs are traversed by delicate growth-lines. 
Horizon and Localities. — This S23ecies is very abundant in the upper 
beds of the Silurian, and has been found in beds of that age in Sweden, 
Norway, England, Ireland, the Urals, and America. The Australian 
specimens have been collected in some numbers in the yellow or brownish 
shales of Yarralumla, Duntroon, and Dangelong. 
' [Now written Str&phcmena . — For reasons for this change see Hall and Clarke, Pal. N. York, 1892, viii, 
Pt. 1, pp. 246-252.— W.S.D.] 
