68 PROCEEDINGS OF THE NATIONAL MUSEUM vol. ii4 



Remarks. — Variation in the series available occurs mainly in size. 

 Males vary from 4.5 to 7.1 mm. in length and from 2.9 to 3.9 mm. in 

 width; females vary from 4.8 to 6.8 mm. in length and from 2.9 to 

 3.8 mm. in width. Color also shows some variation, for though the 

 majority of specimens are black, a few specimens are distinctly brown- 

 ish (teneral); rarely specimens have the elytral umbones and apices 

 spotted light brown. Pronotal punctures vary somewhat in density 

 and occasionally appear faintly annulate. However, the margin of 

 the puncture is normally not distinct, and small nonsetigerous 

 secondary punctures are rare. 



0. alluvius is closely related to 0. knulli and 0. monticolus. It can 

 be distinguished from the allopatric knulli by its nearly impunctate 

 posterior median portion of the metasternum and by its more convex 

 pygidium. In addition, 0. alluvius averages larger in size (about 5 

 to 6 mm.), is usually a more shining black, and has less obvious dorsal 

 setae than does knulli. 0. alluvius can be distinguished from the 

 partially sympatric monticolus by the general lack of small secondary 

 punctures on the pronotum and by the punctate shining, distinctly 

 convex apex of the pygidium. 0. alluvius can be separated from other 

 species of Onthophagus by the duUy shining black color, the well- 

 separated pronotal punctures, the median pronotal protuberance 

 which is at least vaguely evident in both males and females, and the 

 elj^tral intervals with their two irregular rows of small tubercles and 

 alutaceous surface. 



For some time dming the course of this revision we considered 

 0. alluvius and 0. knulli one species. Small specimens of alluvius 

 have the more pronounced dorsal setae that are usual in knulli (which 

 is always smaller). The relative rarity of well-developed males in 

 knulli perhaps indicates survival under marginal conditions. How- 

 ever, as more and more material was accumulated during the course 

 of this work, a constant morphological difference in the metasternum 

 became apparent; this difference coupled with the lesser differences in 

 the pygidium, the average size-difference, and the allopatric distri- 

 bution made us conclude that two species are represented, 0. alluvium 

 is apparently largely limited to the lowland regions of eastern Texas 

 and eastern Mexico, being particularly common in alluvial areas; 

 knulli occm's in the mountainous regions of southern Arizona. 



The habits of 0. alluvius seem to be quite similar to those of 0. 

 texanus. Specimens were most frequently found in cow dung in 

 wooded areas, 



Lindquist (1935, p, 7) stated (under the name anthracinus) that 

 Texas specimens — 



* * * axe found in dung from March to December, and a hundred or more 

 have been counted in a single dropping, A burrow is dug vertically into ground 



