ORTMANN : TERTIARY INVERTEBRATES. 
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2. Pescadores, Rio Santa Cruz ; 1 lower, 1 small upper valve. 
3. Paso del Rio Santa Cruz ; 1 double valve. 
These are typical, broad O. hatcheri. 
4. Mt. of Observation, lower horizon; 12 double, 3 lower, 1 upper valve. 
Four of them are large, rounded, with short and broad area, but the 
lamellae, although a little distant, are more crowded than in the typical 
O. hatcheri. Only slight traces of radial ribs are present. 1 individual 
of medium size has the lamellae as in O. hatcheri , but its form is elongate, 
with long, triangular area. 3 of medium size have the lamellae more or 
less crowded, the form elongate, and the area long. The rest are young 
individuals of very irregular shape. 
5. Mt. of Observation, upper horizon ; 2 double valves. 
One is of medium size, the lamellae distant, the outline elongated-tri- 
angular ; area very long and narrow. Slight traces of plaits. This is the 
first individual that shows traces of crenulations near the area on the upper 
valve. The other is young, ovate-triangular, with a long area. No char- 
acters of O. hatcheri. 
6. San Julian, Oven Point; 2 double, 1 lower valves. 
Both double valves are the typical O. hatcheri , and in one of them — a 
very rare case — the upper valve has radial ribs. The single valve is of a 
more ovate form. All three lower valves have distinct radial ribs ; in no 
other set of specimens are these radial plaits so strongly developed as in 
this one. 
7. Port Madryn, New Bay, Terr, of Chubut; 2 lower valves from 
between tides, 2 lower and 3 upper valves from ca. 25' above high tide. 
One lower valve of the lower horizon is the typical O. hatcheri ; the 
others have the lamellae more crowded ; their outline is ovate or elongate. 
Area broadly triangular. 
8. Shore of Salt Lake, 10 miles north of mouth of Rio Chico ; 1 double, 
4 lower, 5 upper valves. 
Double valve, and 1 lower, typical O. hatcheri , with distant lamellae. 
The rest is more ovate or irregular. 2 isolated upper valves show dis- 
tinct traces of crenulations, in the larger of them the lamellae are remote. 
9. Upper Rio Chalia ; 1 double, 5 lower, 9 upper valves. 
The double valve agrees absolutely with O. bourgeoisi Phil. One up- 
per valve is the typical O. hatcheri , with rounded outline and remote 
lamellae. Another upper valve is the typical O. philippii (= O. bourgeoisi, 
