108 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
but with very long area). The rest are young individuals, of various 
shapes ; one upper valve shows crenulations on the margin ; a very young 
lower one (pi. XIX, fig. T) shows the corresponding grooves. 
10. 30 miles north of upper Rio Chalia ; 2 double, 6 lower, 2 upper 
valves. 
One of the double valves is the type of my O. philippii; the lamellae 
are on ^ of the shell remote, as in O. hatcheri. The other double valve, 
and one of the lower ones agree with this in the elongate form and long 
area, but the lamellae are crowded. Another lower valve is very broad, 
almost circular, area short and broad, and it agrees in these respects with 
O. hatcheri ; but the lamellae are very crowded. The type of O. philippii 
has distinct crenulations near the area of the upper valve. 
1 1. Canon near Sierra Oveja, Rio Chico (interstratified with Santa-Cruz 
beds) ; 1 double, 2 lower, 2 upper valves. 
The double valve is the typical O. philippii , but the area is a little 
shorter (= O. bourgeoisi ). Outline of the others irregularly oval, lamellae 
not much crowded, but less distant than in O. hatcheri. Area triangular. 
In one of the upper valves the lamellae are very distant for half its length. 
12. Shell Gap, Rio Chico, lower horizon; 1 double, 1 lower, 1 upper 
valve. 
The double valve corresponds to O. hatcheri. The lower valve (pi. 
XVII) is elongated, area very long, lamellae crowded, only near the beak 
more distant. The upper valve is broadly ovate, the lamellae more 
crowded than in the typical O. hatcheri. 
13. Mayer Basin, upper lignites; 2 lower valves. 
Elongate, with long area, corresponding to O. philippii. 
14. Arroyo Gio ; 2 double, 16 lower, 1 upper valves. 
One of the double valves is very large, round ; the other one small, 
circular ; the latter with remote lamellae, and distinct crenulations in the 
upper valve near the area ; it is a typical O. hatcheri , while in the larger 
one the lamellae are more crowded. One of the lower valves is very 
large, round, with lamellae like O. hatcheri , but the area is elongated. 
The upper valve is an ovate, typical O. bourgeoisi , with crenulations. 
The rest (15 lower valves) grow in clusters, of very irregular shape; no 
characters of O. hatcheri are present ; area mostly triangular, in a few very 
long; 4 of them show grooves corresponding to crenulations near the 
area ; they correspond to O. bourgeoisi and philippii. 
