2 PROCEEDINGS OF THE NATIOISTAIj MUSEUM VOL.72 



Species that lack the protection of a well-chitinized exoskeleton 

 are more definitely confined to areas where the humus layer in which 

 they live remains moist at all times and so furnishes food and con- 

 tinuous protection. Such areas are most commonly found in the 

 Tropics where the rainfall is usually more abundant than in tem- 

 perate regions. The greater differences between summer and winter 

 climate in the temperate regions doubtless may also be considered 

 as limiting factors of distribution, though the soil conditions appear 

 to be much more important. 



Humus faunas may be limited by the nature of the soil, as well 

 as by the need of continuous moisture. The drying of the surface 

 does not endanger the existence of a humus fauna where the condi- 

 tions are such that its creatures can take refuge in a moist subsoil, 

 but clays or colloidal soils do not afford such protection and are not 

 adapted to the needs of a humus fauna. If the soil cracks in drying, 

 the humus animals may enter, and may seek deeper levels as the 

 season advances, but when the rain comes and water fills the crevices, 

 the soil dissolves into soft mud and creatures buried in it have little 

 chance to escape. Whether for this cause or others still to be recog- 

 nized, the rule seems to be that the colloidal soils have little in the 

 way of humus faunas, and often no animal life, even under condi- 

 tions that in other respects may appear quite favorable. While 

 humus faunas exist in many regions of colloidal soils, it is found 

 in such regions that the animals live in the humus blanket, and are 

 confined to the places where the blanket is thick enough to hold 

 its moisture through the dry season. Under such conditions the 

 animals may be said to live in the humus rather than in the soil, 

 except as the character of the soil may be modified under a humus 

 blanket. 



As with other cosmopolitan orders of millipeds, most of the species 

 of Colobognatha are tropical, but the few that have been found in 

 temperate regions have been recognized as distinct genera or even as 

 distinct families, not represented in the Tropics. From the United 

 States only five species of Colobognatha have been known and in 

 recent years this number has not been increased ; three of the species 

 were described from the Eastern States and two from the Pacific 

 slope. One of the eastern species has been reported from several 

 localities and belongs to the genus Polyzonium^ that occurs also in 

 Europe. The other eastern species, representing two very distinct 

 genera, Brachycy'be and Andrognathus, apparently are rare and 

 local, no two species being reported from the same place. With the 

 addition of the new forms described here, the preponderance in 

 Colobognatha must now go to the Western States, where eight genera 



