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 1 1 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. g/ycimeris.} 



Philippi's P. ibari represents an extremely thick and high individual, 

 while his P. magellaiiicns is extremely transverse. Both are large, and 

 possess accordingly a very 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. n 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. 



