214 
ends scarcely before the middle of the apical half of the wing and 
just beyond the extreme base of the first submarginal cell, the sub- 
costal cross vein next its tip. The prefurca arises at the end of the 
basal third of the wing and is scarcely shorter than the rest of the 
vein. The marginal cross vein is as far beyond the base of the first 
submarginal cell as that from the origin of the third longitudinal 
vein ; the latter is hardly in the least bent at its base where united 
to the branch of the fourth longitudinal vein. The second sub- 
marginal and first posterior cells are of almost equal length and 
longer than the breadth of the wing. The discal cell is subtriangu- 
lar, enlarging toward its rectangular apex from its pointed base. 
Petiole of third posterior cell shorter than the cell. Fifth longi- 
tudinal vein distinctly and considerably bent at the great cross vein, 
the fifth posterior cell less than twice as long as its Inedian breadth. 
The legs are imperfectly preserved, but are relatively very short. 
Length of wings, 9 mm. ; breadth, 2.75 mm. 
Named for Prof. 5. W. Williston, of the University of Kansas, a 
diligent student of American Diptera. 
Florissant, Colorado. Two specimens, Nos. 9312, 12688. 
CLaDURA Osten Sacken. 
Cladura Osten Sacken, Proc. Acad. Nat. Sc. Philad., 1859, 229. 
Cladura is a North American genus and has indeed been found 
only along the eastern shore from Canada to the District of Co- 
lumbia. Loew described a European species, but Osten Sacken says 
it cannot be placed here. ‘Two living species only are known. Up 
to this time it has not been found fossil, but I now place here a 
couple of species from Florissant, differing considerably from each 
other, in that one, a stout species, has spotted wings, very short and 
broad for a Cladura ; while the other, a slender form, has clear wings 
of the usual proportions, nearly four times as long as broad. They 
agree, however, tolerably well in their neuration, but differ from 
modern species of Cladura in that the distance between the base of 
the prefurca and the tip of the auxiliary vein is less than, in the 
stout form hardly more than one half, the breadth of the wing; in 
that the tip of the auxiliary vein lies distinctly before the base of 
the first submarginal cell; that the petiole of this cell is only about 
half as long as the distance between the subcostal and marginal 
cross veins; and in the slight basal arcuation and subsequent 
