1936] The Persius Group of Thanaos 107 
black postmedial spots, and less defined submarginal and 
marginal ones contrast strongly. The pale fringe, empha- 
sized by Lindsey, occurs in specimens of both types of geni- 
talia, and is probably a dry climate modification of all the 
species. 
The chief characters are : left valve with middle lobe 
much widened, separated from the upper lobe by less than 
its width, with a heavy keel on its inner face, extending 
about half way to the tip (present in a single aberrant or 
hybrid specimen of afranius) ; upper lobe with spinulation 
limited to its apical part ; right valve with upper lobe mas- 
sive, separated from lower by a narrow chink, its tip re- 
curved into a massive, heavily spinulated hook, which in- 
variably folds back on itself in the mounted valve, its outer 
lower edge not toothed, more or less lobed and hairy at the 
middle; the whole larger than in any afranius or lucilius , 
or most haptisise , being over 0.8 mm. long with few excep- 
tions, and over 0.62 (%) mm. wide in all specimens seen. 
Lower lobe short and heavy, less than twice as long as up- 
per lobe, its tip ending in a more or less distinct rounded 
thickening on inner side, and wholly without teeth. The 
whole structure, including the tegumen, is more massive 
than the other species. The broad middle lobe on the right 
side can be seen without dissection, but is set on obliquely 
and often seems narrower than it really is. 
I have specimens before me from Bristol (fig. 1) and 
Johnston, R. I., New Haven, Conn., Crosby and Yapyank, 
N. Y., Eagle, Ala., and numerous western specimens, mostly 
from Col., Wyo., Alta, and Cal. The latter are generally 
mixed with afranius, but the massive genitalia will dis- 
tinguish them ; in fact some California specimens are more 
massive than the eastern ones. I have only seen spring 
dates from the east, though it flies in July in the Rocky 
Mountains, so I believe it is single brooded, in the spring 
where the climate permits. 
In the remaining species the middle lobe of the right 
valve is slender, with a wide space between it and the up- 
per; and its inner keel is weak and stops practically at its 
base; the upper lobe shows some tendency for the hair to 
extend down in a line to near the base of the middle lobe 
