ORTMANN: FAMILIES AND GENERA OF NAJADES. 229 
separated by a distinct reéntering angle. In some forms these two 
parts become tuberculiform, and the connecting bars disappear, so 
that the beak sculpture appears composed of isolated tubercles (Unio 
pictorum). A further step in advance is that the anterior part of the 
double-looped bar breaks up into tubercles, and finally into zig-zag 
bars. The manner in which this is accomplished remains yet to be 
studied, but always, in the zig-zag sculpture, the posterior loop, which 
lies upon the posterior ridge, is the most conspicuous part of the whole 
beak sculpture. Thus the zig-zag sculpture is the most extreme con- 
dition of a line of development, which goes from the simple concentric 
bar, through the double-looped, to the zig-zag condition. In certain 
forms with zig-zag sculpture, all three stages are clearly present on the 
same shell (Nodularia douglasie), and possibly this may be observed 
always in such cases.° 
Entirely different from this is the radial sculpture, but I have reason 
to believe that it also goes back to the concentric type. At any rate, 
I have seen in specimens of the genus Lamellidens that there are also 
originally one to two simple concentric bars. But after these only 
the lateral parts of the bars, which anteriorly and posteriorly curve 
up toward the beak, are developed, while the middle part upon the 
disk becomes obliterated. Then these lateral parts, which have a 
direction from the beak toward the basal margin, are emphasized, 
developing more strongly, and their direction remaining a radial one. 
Since there are two groups of radiating ridges (an anterior and a 
posterior), the median ones naturally must interfere with each other 
upon the middle of the disk, and must come in contact there at a more 
or less sharp angle, when fully developed. This is in fact the case, 
wherever we see radial sculpture well developed. There are always 
two sets of radiating folds or ridges, one originating in front, the other 
behind the beak, which cover the disk interfering with each other in 
the middle of the shell. In some cases, however, this is not very clear, 
and such cases possibly present the highest type, with the original 
features obliterated. The radial sculpture is another extreme standing 
at the end of a line of development starting from simple concentric 
loops. 
These conditions are worthy of being studied more closely. In regard 
5 In certain cases it seems that double-looped sculpture may again be simplified 
by the re-entering angles becoming less sharp and only sinuate in the latest bars. 
But this is surely a sign of the incipient obliteration of sculpture. 
