the Coleopterous Genus Hexodon. 597 



as indicating specific distinctness^ wliile the amount of 

 ditrorenco, when the marked constancy of this character is 

 contrasted with tiie no less marked inconstancy of the ex- 

 ternal form of these beetles, must be held to show much more 

 clearly than the hitter the relationships existiiij^ between the 

 species. We have here, in iact, a structure which is both more 

 plastic, as shown by the extraordinary diversity of its forms 

 olten to be found in species undoubtedly very closely related 

 to one another, and more stable, as seen in its constancy of 

 shape wiien examined in a series of similar individuals, than 

 any external character, and which by its nature is the best 

 anatomical indicator of the possibility or otherwise of inter- 

 breeding. Reliance upon it will undoubtedly produce results 

 sometimes conflicting with ideas of species which have been 

 based only upon external features, and it may be necessary, 

 in the absence of any other term, still to describe as " species " 

 forms Nvhich show marked external differeiices without 

 apparently any corresponding genital characters; but still 

 more must we admit that, where distinct forms of genitalia 

 occur, even if no external difference can be found (and I 

 believe such cases to exist), these must be accepted as 

 indicating distinct species. 



In the present genus there is a concurrence of characters, 

 internal and external, in all the forms 1 have distinguished, 

 and it consists, so far as at present known, of a group of 

 five exceedingly closely related and probably lately evolved 

 S])ecies, forming the reticulatum group, and three more 

 ancient and less closely interrelated species, forming also 

 a natural group (the uidcolor group). 



The following synopsis gives tlie most salient characters 

 of each species : — 



1 (10). Posterior angles of the elytra sharp. 



(reticulatum group.) 



2 (3). Body very convex, with scarcely 



flattened elytral margins 1. montandoni, Buq. 



3 (2). Body not very convex, with dis- 



tinctly flattened elytral mai-gins. 



4 (7). Hind angles of the pronotum 



rounded. 



5 (6). Apical angles of che elytra slightly 



produced 2. reticulatum, Oliv. 



6 (5). Apical angles of the elytra not pro- 



duced 3. patella, sp. n, 



7 (4). Ilind angles of the pronotum sharp. 



8 (9). Lateral margins of the elj^tra 



narrow ; elytra together scarcely 



broader than long 4. (jriseoscricans, Fairm. 



9 (8). Lateral margins of the elytra broad ; 



elytra together distinctly broader 



than long 5. latissimum, sp. n. 



