ORTMANN ! TERTIARY INVERTEBRATES. 
95 
broad, flat intervals ; they are crossed by fine concentric lines of growth. 
Area triangular, increasing in width with age, with rhombiform sulci up 
to 6 or 7. Hinge teeth up to 11 on each side ; the median ones, in old 
individuals, covered by the dilated area, so that only 4 to 5 on each side 
are visible. Only the median teeth slightly geniculate, the lateral ones 
oblique to horizontal, but straight. 
Remarks : As v. Ihering has already pointed out, the external form of 
this species is extremely variable. Sometimes it is perfectly circular, but 
often more or less oblique, or even transversely elongate ; the latter ex- 
treme is represented by Philippi’s P. magellanicus. We possess from 
Punta Arenas all intermediate conditions in shape. Also the thickness 
of the shell is variable. (As to variations in shape and thickness in this 
genus, compare Wood, 1856, p. 68, under P. glyeimeris .) 
Philippi’s P. ibari represents an extremely thick and high individual, 
while his P. magellanicus is extremely transverse. Both are large, and 
possess accordingly a veiy broad area, which leaves only the 4-5 
lateral hinge teeth exposed. The same number of teeth is present in our 
largest individuals (see below, No. 11 and 12). We have a smaller indi- 
vidual (No. 3) that corresponds in shape exactly to P. magellanicus, but 
has a larger number of hinge teeth. V. Ihering figures a very large indi- 
vidual of subcircular outline, in which the median hinge teeth are not 
completely covered. 
I give here the measurements of some of our specimens, compared with 
those of Philippi and v. Ihering. 
No. 
Height. 
Length. 
Diameter. 
Anterior hinge teeth. 
Sulci. 
Shape. 
I 
48 
50 
18 
1 1 
I 
almost circular (left). 
2 
50 
54 
20 
9 
? 
oblique (left). 
3 
66 
80 
2 7 
9 
? 
very oblique, transverse (left). 
4 
70 
ca. 70 
2 5 
8 
? 
almost circular (left). 
5 
70 
72 
30 
12 
? 
subcircular (left). 
6 
72 
7 1 
26 
10 
3 
almost circular (right). 
7 
74 
77 
3 2 
12 
? 
moderately oblique (left). 
8 
76 
77 
3i 
8 
p 
almost circular (right). 
9 
82 
92 
33 
8 
4 
oblique (left). 
10 
87 
ca.95 
35 
6 
? 
moderately oblique (right). 
1 1 
9 ° 
85 
33 
5 
5 
moderately oblique, high (left). 
12 
92 
100 
38 
5 
6 
oblique (right). 
13 
90 
1 16 
37 
5 
7 
very oblique, transverse. 
H 
98 
92 
ca. 40 
5 
? 
moderately oblique, high. 
IS 
99 
106 
38 
ca. 8 
4 or 5 
subcircular. 
