Ortmann; Families and Genera oi Najades. 229 



separated l>\ a distinct reentering angle. In some forms these two 

 parts become tuberculiform, and the connecting bars disappear, so 

 that the beak sculpture appears composed of isolated tuben les 

 pictorum). A further step in advance is thai the anterior pari 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 douglasia), and possibly this may be observed 

 always in such cases. 5 , 



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 



s In certain cases it seems that double-looped sculpture may again be simplified 

 by tin- re-entering angles becoming less sharp and only sinuate in the latest bar-. 

 But this is surely a sign of the incipient obliteration of sculpture. 



