230 CALIFORNIA ACADEMY OF SCIENCES. [74] 



The sequence of genera in the above tabular statement is, 

 it must be confessed, unnatural in approximating Stilicus 

 and Prederus, these being undoubtedly widely divergent 

 forms. It merely serves to show, however, that it is im- 

 possible to present in a linear arrangement, groups com- 

 posed of elements which are divergent from one or more 

 central types, and which can only be represented graphi- 

 cally b}^ the diagrams adopted in chemical science to exhibit 

 the structure of a compound molecule, the various affinities 

 being shown by connecting lines. 



If a linear arrangement be pursued, based upon the mod- 

 ification of any special organ or part of the body, similar 

 breaks must inevitably occur. Assuming, as above, that 

 the structure of the presternum is of more importance than 

 that of the tarsi, the latter being in turu of greater moment 

 than that of the labrum or mandibles, we should isolate 

 Paederus as a group intermediate between the Lathrobii 

 and the Sunii, and it would not be consistent to separate 

 them by the the latter group, although it may include forms 

 which in a radial arrangement would be brought very near 

 certain types of the Lathrobii. Such for instance are 

 Stilicus and Echiaster, in distinguishing between which the 

 prosternal character loses some of the importance which it 

 is supposed to possess, unless we regard the similarity of 

 habitus as a mere coincidence. The latter I have assumed 

 in the case of Stilicus and Scopaaus. 



Notes. 



Ababactus Sharp. — This genus is represented in our 

 fauna by A. nactus Horn, and A. pallidiceps Cas. 



Trachysectus Cas. — Represented by T. confluens Say. 



Caloderma Cas. — Recent investigation shows this genus 

 to be similar in prosternal structure to Medon, from which 

 it is distinguished by several important characters. The 

 labrum is short, small, conical, very feebly explanate near 



